One problem might be that the riser wasn't uniformly cut and is rather
irregular/jagged/bent.

True 'dat. More importantly, the drill string is still inside the
casing/riser. Picture a 10-12" diameter steel casing with a 5"
diameter steel pipe inside. They tried to sawcut them both, but ended
up snipping through both with massive shears. Hence, both the riser
and drill pipe are pinched and ragged.

-Zz