What historical circumstances gave rise to the Crusades?

The circumstances probably date back thousands of years.

There is no doubt that for around 3,500 years the Egyptian pharoahs were the overlords of Nubia, Libya and the Levant.

Dating back to 2000 BC one of the most powerful administrative centres in Canaan was the port of Byblos, which was pretty much an Egyptian colony which made a living from controlling the imports and exports going between Egypt and Canaan.

"Watson Mills and Roger Bullard suggest that during the Old Kingdom of Egypt, Byblos was virtually an Egyptian colony.[16] The growing city was evidently a wealthy one and seems to have been an ally (among "those who are on his waters") of Egypt for many centuries. First Dynasty tombs used timbers from Byblos. One of the oldest Egyptian words for an oceangoing boat was "Byblos ship"."

https://en.wikipedia.org/wiki/Byblos#Egyptian_peri...

In 1500 BC the administrative centre for Egypt in Canaan was Gaza.

".[86] During the Late Bronze Age (1550–1200 BCE), large parts of Canaan formed vassal states paying tribute to the New Kingdom of Egypt, whose administrative headquarters lay in Gaza.[87]"

https://en.wikipedia.org/wiki/Israel#Antiquity

However there was a period known as the Hyksos, when through royal marraige foreigners came to power in Egypt, and they ended up being expelled from Egypt as they upset native Egyptians when it became apparent that they were trying to "Usurp" things Egyptian.

"Rather than building his own monuments, Apepi generally usurped the monuments of previous pharaohs by inscribing his own name over two sphinxes of Amenemhat II and two statues of Imyremeshaw.[16] Apepi is thought to have usurped the throne of northern Egypt after the death of his predecessor, Khyan, since the latter had designated his son, Yanassi, to be his successor on the throne as a foreign ruler.[17] He was succeeded by Khamudi, the last Hyksos ruler. Ahmose I, who drove out the Hyksos kings from Egypt, established the 18th Dynasty.[16] ".

https://en.wikipedia.org/wiki/Apepi#Reign

It is even possible, that the person that led this Nubian uprising against the foreigners, actually got usurped by the foreigners, that later turned him in to Moses, a hero of the Canaanites, that returned his people to Canaan, when he was actually a Nubian, that returned his people to Nubia, after they disposed of the Hyksos.

Though this is just a theory that is likely not widely accepted.

Now along with those foreigners that fled Egypt to escape the Ahmose I's uprising, were Israelites that settled in Canaan, and they appear to have set about conquering Egyptian rule, not by military conquest, but by recognising God, over the Pharoah, as the true ruler of Canaan, and of course Jerusalem became the centre for this religion, that seeked to replace the pharoah with God, and replace Byblos and Gaza with Jerusalem.

".[88]:78–79 The Israelites and their culture, according to the modern archaeological account, did not overtake the region by force, but instead branched out of these Canaanite peoples and their cultures through the development of a distinct monolatristic—and later monotheistic—religion centered on Yahweh.[89][90][91][92][93][94] 

https://en.wikipedia.org/wiki/Israel#Antiquity

The Egyptian empire slowly went in to decline, and in 30 BC the unthinkable happened and a Diva named Cleopatra used her sexuality to try and sleep her genes in to world domination and she two timed Ptolemey with Julius Caesar and the result was a baby boy named Caesarion.

However, Caesarion would unfortunately become an illegitimate child of Julius Caesar's, and Julius Caesar would not recognise Caesarion in his will, instead leaving the Roman empire to his adopted son Octavian.

Lucky for Octavian Caesarion was still only 17 years of age, but Caesarion did make his way back from India to Egypt to take up pharoahship, and likely had removing Octavian from the Roman throne in mind, as afterall, Caesarion was in fact the rightful heir to the Roman throne, not Octavian.

However before Caesarion could get any plans in to action, Octavian, a little older than Caesarion, had the good forsight to simply have Caesarion executed and the 3,500 year old Egyptian empire dissolved based upon the unpaid debt they owed Rome.

"Ptolemy XV Philopator Philometor Caesar (Koinē Greek: Πτολεμαῖος, Ptolemaĩos; 23 June 47 BC – 23 August 30 BC),[1] better known by the nickname Caesarion (Καισαρίων Caesariō), was the last pharaoh of ancient Egypt, reigning with his mother Cleopatra from 2 September 44 BC until her death by 12 August 30 BC, and as sole ruler until his death was ordered by Octavian, who would later become the first Roman emperor as Augustus.

Caesarion was the eldest son of Cleopatra and possibly the only biological son of Julius Caesar, after whom he was named. He was the last sovereign member of the Ptolemaic dynasty of Egypt."

https://en.wikipedia.org/wiki/Caesarion

Now, back then, an Egyptian pharoah being executed by Romans, and the 3,500 year old empire being dissolved, was "no small thing".

Coincidentally it coincided with the rise of Abrahamic religion that would spread throughout the world, and anti-roman christian campaigners would appear in Rome.

Roman emperor Claudius for example had to expel Christians from Rome as early as 40 AD as they were causing mayhem at the instigation of a rebel leader named Chrestus.

They were likely trying to convince Roman citizens that these Roman emperors are not the true emperors.

"It is also reported that at one time he expelled the Jews from Rome, probably because the Jews within the city caused continuous disturbances at the instigation of Chrestus.[ii]

https://en.wikipedia.org/wiki/Claudius#Religious_r...

But what the fall of the Egyptian empire would do, is it would leave a void that is there to be replaced, and three competing religions would fight over it.

Though, religion would have it that the power base was Jerusalem and not Egypt, as it appears history got lost.

1. The Muslim destruction and desecration  of Christs tomb and destruction of the Christians HQ Holy Sepulcher Church in Jerusalem under Muslim caliph Al Hakim Mansour year 1009-11.

2.) The Seljuk Muslim Turks taking Anatolia (Turkey) from the Christians cutting off the Christian route to the Holy Land entirely.

3.) Too many Christian knights trained only to fight and kill stomping around Europe with nothing to do.

So, Pope Urban year 1095 called for Crusade because of these circumstances.   

 What historical circumstances gave rise to the Crusades?