This has happened to me and my friend to it's seems to be at it's worst when a house alarm goes off. It's when a lot of zombies agro on to you the get stuck aggroing to you. I tried to leach off my friend aggroed Zed's, grab a couple, lead them away but when they loose interest in me, they lock on to my friend who is already on the other side of town. Sadly plays.tv didn't record the senario. We usually burn them off but pretty difficult since the longer it last the more zombies get stuck to you. We'd do this more then once and it got pretty annoying. We once took a hoard 30+ half way to West point from Muldride, run back to Bob and Kate's house and in 4-7 minutes they would come from back for us trapped us on the second floor and we died.
It's really feels like they are perminently stuck aggroing at least one of us just always walking towards one of us.