Why does my dog's hair on his back stand up when he...?

Hackles express excitement of any kind, not just aggression.

I also have a Beagle and he raises his hackles when he meets a new dog, when he chases squirrels, when the dogs around him (particularly at the dog park) are getting loud and playful, and even when he's just enjoying a good romp with one of my other dogs.

In order to determine whether a dog is about to display aggressive behaviors, you must examine the posture of his entire body, not just his hackles.

What you described is signs of aggression, When the hair goes up, usually the tail points upward, too. If this is the case you must calm him down, without frustration or nervousness. Stay calm, and confident at all times, but correct him before it escalates. Have the other dog smell his rear, and if your dog is willing, have him smell the other dog's rear. This is social behavior among dogs, and don't do a face to face with the dogs. You are the one he looks to for positive direction. So, keep your energy calm at all times, and confident. This is how you get your dog to trust you. Never let unwanted behavior go by without a correction.

LOL! My dog does that too. The reason their hair stands up is to make them look bigger and more intimidating when they go into alert mode.

Those are the hackles,the only time I have ever seen a dogs hackles stand up is when there about to fight.

Its a dogs way of making themselves appear to be larger to there opponent.It can also be fear based as well.

My dogs hair does this when he is excited or smells other animals (even cats smell in our yard). He has never been aggressive in this posture. He also snorts like a pig. I believe it is his hunt/alert mode.

Sign of aggression or fear, if your dogs hair is standing up you need to remove him from this situation ASAP. Your dog is severely stressed, and you could be looking at and aggressive situation

most likely he's scared, or nervous?

my dog usually just barks like crazy...

maybe your dog just doesn't like other pupss