Fishing wide open flats from a boat, for striped fish. I'm usually stripping line like I am trying to start a chainsaw with bad gas and the hits are usually pretty violent. Like I tried to explain earlier in this thread (but it's hard to describe in words) I believe it happens when they engulf the fly at breakneck speed and the hook point hangs up at a severe angle to the direction of line pull as the hook is going INTO the fishes mouth.... then they either turn and blaze off....or I set the hook, and....Snap! In this situation it is trying to "roll the hook out, but they are so sharp that the point sticks putting some pretty severe sudden tension on the hook bend.
I know this happens alot with whites and hybrids because It is not rare at all to land a fish on a 34007 that is stuck somewhere in the lower jaw and also notice a nasty fresh tear in the roof of its mouth. Or land one stuck in the upper part of his mouth with a torn up bleeding tongue. The 34007 is strong enough to resist flexing due to the wire diameter, and isn't so heavily tempered that it becomes brittle.....like some ultra-tempered fine wire hooks are.
I have also broken several B10S when lightly hung up on a piece of wet wood and I throw a brisk roll cast over the stick to free the fly (ya know, that little trick that we all try a couple of times before we go fetch it manually).
Just the sudden jerk of a loop of 7-9wt. Line will snap a #4 B10S. I can break one without ever pulling hard on the snagged fly, just by doing a fast tight looped roll cast if the point is stabbed into soggy wood, so that is what convinced me that it was a "line pull angle vs. brittle tempered steel" thing.
Does any of that BS make any sense? LOL