Simple rule: your finger doesn't enter the trigger guard until your sights are on the target.
Cops are the worst offenders of this rule, judging by the number of reports of them shooting themselves while holstering or unholstering their sidearms.
I don’t know exactly what happened, since it was never completely reported. However, I doubt what you speculate since there were 30 to 40 rounds fired over the berm. I cannot conceive of 30 to 40 rounds being AD’s.
My guess (and it is entirely a guess since a report was never publicly released) is that they were playing with the new fully-automatic M-16’s they got from the Feds and placed some targets on top of the berm instead of in front of it.