Why does he act the way he does?

Save marriage with Counselling.

Only he can explain his behavior, but it seems odd you would have this kind of question and not think to ask him what is happening. That suggests you may have a communication problem that should be addressed without further delay. Talk with him, explain your concern, and ask him what is happening.

Here's what my husband does if there's not enough salt in the food to his liking: He picks up the salt shaker and puts salt on his food. Nothing more, nothing less.

If he happens to wake up if I go to the bathroom at night, he'll ask if I'm okay, and when I say yes, he'll go straight back to sleep.

See, this is your perfectly normal baseline and don't let anyone tell you otherwise.

Your husbands behaviour is out of line, and no, you are not to blame. It sounds like he's looking for excuses to pick a fight. Since you say "lately", I'm going to assume that this is new behaviour from him, so something seems to be going on. Have you tried talking to him about it?

EDIT: If he doesn't want to talk about it and blames you for everything, then it's going to be very difficult to resolve, and I think you need to decide what your next move is going to be.

How long has this been going on? It's one thing to stick it out while you go over a hump, it's another thing to stay in a mentally abusive relationship.

You are not to blame.  This is the precursor to domestic violence.  He feels entitled to something. And entitled to use anger to try to get what he wants.  It is difficult but it needs to be stopped. He doesn't have a right to be angry.  That is an immature and childish response.  Occasionally there can be other stresses which make him particularly sensitive.  In which case those stresses can be addressed.  But you need to make it clear that anger is unacceptable.  If you need to go to the toilet then you need to and there is no room for complaint.  He could add salt to the food.  What if you PREFER less salt?  Did you make the dinner for him?  So if he complains then tomorrow ask him to make the dinner and do it how he likes it.  Suggest that in future you might take it in turns.  I can only repeat that unless this is addressed now it has a tendency to become a serious threat to life and to limb over time.  Don't let that happen.