Rich-\
a pretty lady!!

I don't think it happens when the hook slides up the leader. If that was the case after the fish took the bait/lure the hook would be pulled into the side of the face opposite the angler. The fish would be hooked similar to those hooked while flossing or fishing an egg bead pegged several inches up from the hook.

Rather I think what happens is that the fish changed it's "mind" at the last minute and snagged itself. Basically the fish sees the bait/lure from a distances, makes a rush and at the last minute turns hooking itself. This accounts for the hard take. It occurs when there is good visibility in the water but rarely (ever?) in dirty water (less than a foot); if it was a case of a droopy hook water conditions would not make a difference.

In clear water I have watch fish make a move towards the fly (usually while watching from a high bank) and turn at the last second. The shock-wave cause the fly to move or tumble and the angler may feel a slight "pluck" even though the fish never took. If the fish turns away and up its chin is exposed. . . . Just a theory based on observation.

Tight lines
Smalma