One of the first jobs I did professionally was the investigation of a failed Kenter link from a large tanker. I learned then that if the anchor is deployed on these vessels when moving at more than half a knot the anchor or chain will inevitably break. I am guessing that the Amoco Cadiz was doing a fair old speed when they tried this final measure.
March 16th was the 40th anniversary, on a TV documentary they were discussing the anchor, they showed -barely visible in the photograph- that one of the flukes had been practically torn away, and the remaining fracture surface was smooth, almost polished, maybe the anchor had had a lot of friction against the sea bottom ?