[GAMEPLAY] Top Lane Champs Bug

I have noticed the last few games that the top lane champions specifically have been acting "stupid". They will just sit there and let you kill them. They will not just sit in turret fire, but if you are actively hitting them, they will just stop and not attack, making themselves a sitting duck. I was wondering if anyone else has had this problem. (Specifically I have seen this with Amumu and Darius).