Minions not taking tower agro

I had this happen multiple times. Most the time I just took it as some freak fluke. But when I'm in the middle of a tower push and minions already been taking shots while I been auto attacking away. Then I all of a sudden get shot in the face with 4 minions still under tower. I'm gonna call Bs bug. The whole enemy team was mid so there was no way for me to shift agro. This seriously ruins a lot of my tower pushing.