I've always wondered the same about beef cattle and dairy cattle bulls. Why the dairy ones are so much more aggressive. Beef bulls are very docile and a dairy bull is very unpredictable. You'd think it would be the other way around. There was a little Jersey bull that used to get out of his pasture down the road and he was aggressive to humans.That's one reason that the dairy farmers have gone to artificial insemination
Jersey bulls have probably the worst reputation and even seem to develop the aggression younger than some of the other breeds. If someone is letting a jersey bull break free they are NOT being good neighbors. They can be seriously dangerous.
Funnily enough the ones raised around people, and with the most contact seem to be the worst of all. They don't learn to be afraid of people when they are young as they would with the herd.
It wasn't all that uncommon to hear about people getting hurt really bad, some killed by bulls. People who should know better. The problem that signifies is they are unpredictable, as you said, to even people who should know better.
A I isn't the answer to all your problems. Most owners with medium or larger herds keep a bull or two even if they AI. They are better heat detectors than humans.

If you have a bull that's gotten aggressive with you, best thing is to get rid of him. He WILL do it again, as it means he's lost fear of you.
I learned to respect big Sows on pig farms even more. If you don't know what you are doing , stay away! They are quicker than bulls, or any cows or horses to smell fear and will take advantage of it
I can do without Hogs. I don't eat much pork anymore, though a really good ham is difficult to turn down. Joe