it tends to happen on biceps that the hand is rotated open. this places immense stress on the lower biceps tendon on the inside of the elbow. any small bend in the elbow places more stress on this tendon. POP GOES THE TENDON.
The short answer is poor technique. one should keep their arms STRAIGHT - NO BEND IN THE ARMS WHATSOEVER.
i know 3 guys who have torn their bicep doing deadlifts with an over/under grip.
one guy tore his right off the floor because yanked the bar with his arms to help get it going, bad idea.
this is why i deadlift double overhand now, it's much more difficult but your grip will catch up in time.
Good luck with your recovery.