Interesting bit from the report (often what is chosen to include in a preliminary report can be revealing):
The ground handling agent accordingly turned
off ground power at the stand’s control box but left the
power umbilical cables attached. The engineer visually
confirmed on the flight deck that ground power was no
longer available.
Given prior "sneak paths" in the electrical system one wonders if having the cable connected but unpowered resulted in an unforseen circuit.
I have no idea exactly what turning off the power at the stand does, all conductors disconnected, all grounded or some other configuration.
Does sound liket the ELT is involved though, of course there is still the chicken and egg question. (Which came first)
Think about that long enough and you end up with a chicken ommellete...