That cluster mount is what we call a banded cluster mount , we have used some of them years ago on 3 pot banks where the transformers were above 167 kva , I have never seen a properly installed cluster bracket slide down the pole , I suspect the bolts were not tightened enough which let it slide down a little , if that's what happened, the stinger coming from the cutout might not have been tight in the high side bushing of the pot and the load finally caused it to burn off, hard to say for sure just looking at pictures.