Why do adult sons bully and try to degrade their single mothers?

They don't. Yours may, but I know plenty of adult sons that do not do this so you can't group them all together simply because of the actions of one.

Now if your son is doing this, nobody will be able to give you an answer as to why unless we know a little bit more. Your question is entirely too vague for us to really answer it.

I would have said exactly what odinn7 said, but will try to add some theories of mine.

Some single mothers with one son tend to hang on to them and turn them into the man of the house. The sons tend to fall into a vicious cycle of wanting to be the man of the house, yet also be an adult who needs to be out in the world on his own.
He gets caught in the comfort and ease of having a mother who provides all the domestic things, not to mention the home, and he wants to feel like he helps out, but he mostly feels like less of a man by not leaving. So hate (really love all crusted over with frustration and resentment over the dilemma) takes over.

Variations occur depending on whether mom wants him out of the house or not. If she does, and he has become too lazy about being a self-supporting adult, he bullies her out of the inability to face the fact that he should have prepared himself for being on his own a long time ago. And he blames her, which is partly her fault. If she doesn't want him out but wants him to stop bullying, she needs to realize that she can't have her cake and eat it too. He needs to leave, and she needs to be self-sufficient for everything from company to fixing the toilet.

That's the son living at home. The son who isn't at home is a little different, but not much. And in those cases, the mother is often too dependent on him, and acts too needy.

I have the same problem with my son that is the only one left at home. He wants to hang on and 'bully' me in a sense. I can tell he is trying to be the adult man of the house. Then his inadequacies (me not putting up with it) get in the way of himself and he ends up bullying.
Then on the other hand he is frustrated because while he wants to be the 'head of the house' being the male he also has the dilemma of being under my roof when he wants to be independent. You can't let it go, no matter how much it hurts him you have to put your foot down and YOU make the rules #1 NO bullying!