This an occupational safety issue and does not come under safety of flight ... yet.
Lompaseo, I think it is very much a safety of flight issue. If, as is being reported, there were passengers or crew already on board for the purposes of flight, under the ICAO definition it becomes an accident (more than 5% burns, broken bones etc), so it should turn out to be an NTSB investigation.
Agreed though, occupational safety issue as well.
Why is ground handling the only part of our industry that is not regulated?