I get this whenever I squat with a straight bar. I think my problem is shoulder mobility, and as a result my bicep ends up doing a lot of work to hold the bar on to my back. Just recently I've tried to stretch my shoulders out before squatting and I haven't had the problem (yet). Squat days aren't bad, but the subsequent bench days are brutal.
I know of a lot of others lifters that have this, as well as people on this board ... Dave Tate talks about it too: