I'd like to emphasize that this is not any sort of claim of scientific understanding.
Several years ago, a somewhat portly but shorter (I'd guess around 185 lbs., 5-7) rider I knew had his seatpost snap. The jagged end caught in some loose clothing, and as he tumbled to the ground he was pulled towards that jagged end.
The injuries (and years-long ramifications) he suffered were gruesome.