why did my ex boyfriend do this?

Well one he's probably not over you, so seeing that you followed him and still care to see what's going on in his life. He probably followed you to see how your life is and just to check up on you. And after he saw everything he probably realized how hard it was seeing everything again and that it could possibly lead to something. So out of respect for his girlfriend and the fact he probaably didn't want her to see you were following him and he was following you he blocked you. Now is the point where he'll try to really move on, but he may do this again a coupel times

He probably thought it was ok but the girl said no. Or he did it by accident or something.

His new girlfriend probably told him to. I wouldn’t worry about it.

If you knew he had a girlfriend why did you follow him to start? Maybe she went through his device and put the blocks up? You should have respected his new relationship.