To me, and I have absolutely no credentials to back it up, it sounds like you're squeezing the shit out of the bar with your whole upper body. Like maybe your wasting energy by contracting muscles that should be relaxed so energy can be used more efficiently in the muscles doing the work.
I wonder if you have the proper grip on the bar for DL? Do you grip the bar at the first joints on your fingers as shown in SS or do you grip more in the pad of your hand?(This would cause calluses to form on your hand)
If you grip with the pad of your hand, i'd be willing to bet that switching to the lower grip(more of the fingers than your hand) would solve it. You'd be much less likely to "squeeze" your arms together that way.
Also, make sure your arms aren't bent. If they are that will allow for more chest contraction since you would be able to turn your elbows out a bit.
But, there really is no telling. I'm sure there are a ton of ways this could be happening, lol.