This happens to me once in a great while. You can even see it happen in my pvp vid, while at SM I targeted the one person but the person next to him also got dotted. , I'm running all hardware as well. I think its more or less lag. Once in a while a toon or two will not switch targets, but targets something next to the intended target. Also might be positioning, maybe that target was in range, while the other one wasn’t.

Could be a number of things. I’ll stick with the easy answer of lag =)