how did my cat develop a tumor?

I recently lost my cat, Snickers, that I've had for eleven years. He was probably 15 or 16. I'm not sure. He had been losing weight for a month or two maybe, and I noticed how skinny he was about three or four weeks before he died. He could still eat, drink water and use the litter box so I wasn't too worried. I just thought that him getting skinnier was a part of him getting old. About two weeks before he died, he stopped eating on his own so I had to start feeding him directly, like one piece of kibble at a time. That worked for about six days, but it really wasn't helping. I thought he was probably tired of eating the dry food, so I got him to eat wet cat food. He seemed pretty excited and adamant about the wet food and was kind of acting normal but that only lasted one day. The next day he wouldn't eat anything and I noticed that he only wanted to drink water. It finally dawned on me that maybe there was a sore or something on his tongue or something on his gums that was the culprit. As a lot of people know, it's not easy to get a cat to open his mouth. And you sure can't tell them, "say ahh and stick out your tongue for me." So as best as I could, I tried to take a look inside his mouth and I noticed a red dot that was probably a sore on his tongue and also saw something gray under his tongue, and also his breath seemed to smell horrendous. Usually a cat's breath isn't that great but this was really bad. When I told my mom that we needed to take him to the vet, on the next Saturday, I put Snickers in a box and put a towel over him and we went to the pet clinic that we usually go to. I noticed that Snickers was kind of wobbly in his walking and he had lost so much weight he was like lighter than air and he was just really weak. When we were seen by a vet, she said that Snickers had a tumor under his tongue and also that it looked like the tongue was completely ulcerated and that the tumor was infected. We finally came to the decision that putting him down would be the kindest thing to do. I don't know how he developed a tumor on his tongue, and I think if anyone could say maybey how, I could get a little closure. I am absolutely devastated and miserable because he's gone. I hope someone can maybe even take a guess in how a cat develops a tumor on his tongue and why it was ulcerated and got infected. thank you :D