Really, a good lesson for all DG hunters here.
Sounds like a very unusual/unpredictable situation there.
Not only does a leopard attack/charge transpire in a few scant seconds, but this sounds like it was very much a surprise as well taking the group off guard.
I suspect that there is a certain security when using dogs - the expectation being that the dogs will alert you to the leopard's presence and location. For some reason, it didn't work out that way. Had the party known the approximate distance and direction of the leopard, their formation would have been much more conducive to safe and effective fire as well as not being strung out as they were.
A good rule is to know where everybody is at all times - not easy when walking in-line, especially short behind tall. Remember elementary school? Just walking down the hallway in single file, you can't see a damn thing beyond three people ahead.

This whole account sounds like a very unfortunate accident. Fortunately, nobody was killed, but statistically, this is bound to happen sometime - and occasionally does.
IMO, this is much more forgivable than the number of accidental hunting deaths which occur here in the US every year.