They're called "ejector crabs". Video examples below:
View: https://www.youtube.com/watch?v=pH2k_026KrY
View: https://www.youtube.com/watch?v=5XfSr2foKjQ
If your oar goes into the water at the beginning of the stroke (the catch) at a funny angle the movement of the water will push the handle into your chest and it you can't get under it, you'll just get pushed out of the boat. The guy it the 1st video caught the "crab" because of poor technique. I think the guy in the second video was adjusting his shoes or something and let his oar dig into the water.
From what I saw the rower at the 2 seat in the Cambridge boat was unlucky and someone from the Oxford crew clashed oars with him at a very unfortunate moment just before he was taking the catch which caused him to lose control and catch a "crab".