It's possible that it could be a form issue, but it's unlikely due to the fact that the pecs are supposed to be used during deadlift (along with the lats) to pin the bar close to the shins as you're lifting off the ground.
What most likely happened is that your pecs were fatigued or tight for some reason, and the weight may have jerked away from the body as you were rising and it may have strained your pec.
This may help thusly
http://www.eatmoveimprove.com/2010/0...uscle-strains/