I think it's an odd human nature thing. I get similar all the time, I'll be on the MUP and stop for the cross-traffic. Invariably while I'm waiting someone will come up from behind and darned if they don't frequently pull around and stop in front of me. Get a break and there they are in the freaking way.
I'm pretty quick off the blocks, and I'll usually run right up on the back of them and yell "You wanted to be in front, now MOVE!!"
-R