I think everyone "over-thinks" it and you left out the most important element, which is proximity.
It's like the chubby girl at the bar that you normally wouldn't talk to, but being that you're at a bar, had a bit to drink, now they're suddenly more attractive... Yeah, we all know what happened.
I think the same thing goes for fish. The more time you spend closer to them the better your chances are of them having a lapse of judgement. There's times you can connect the dots and say, "the mayfly hatch was happening, so that's why they were so on my whopper plopper... there were mayfly scuba, splash, sex parties going on everywhere!". Or it could be that the hatch had them all close to the surface feeding and that was why the bite was on.