I have had this happen on multiple occasions. On a 3 blaster team, my elec/elec and 2 nrg/nrg blasters, we all fired on a mob, and the aggro'd mob ran AROUND the person up front, and they all piled on me, with predictable results.
I've been attacked when I've taken no action, and the fight was in full progress, someone has peeled off the fight, turning around, to attack me.
It's happened enough times that it's almost a joke now among my regular teams.
We're pretty sure one of the pranksters in the Super Group has planted an aggro-magnet somewhere in my costume, but I haven't found it yet.