This may be an article that will be useful to you, if you are into such a thing. I see this with some regularity in the clinic with individuals who bench or overhead press. I highly suspect it is caused by the bar moving in the hand during the execution of the movements. Seems to respond well to cueing of getting the bar set firmly in the hand, and wearing wrist wraps appropriately.
Mechanical Strain at the Pisotriquetral Joint - PubMed