Here are a few 'triggers' that I've actually seen trout take with consistently: Cigarette butts tossed off of a bridge; While standing on a bridge/overpass, a bunch of flies all types, sizes and patterns flew out of a fly box and into the water. As they floated down the river, unencumbered by any attachment, every one was taken; I've watched underwater videos where trout take in all kinds of debris. If, after all these observations I had to identify 1 'trigger' it would be that the food source, whatever it was, was completely free in its float and looked natural to the trout. Had nothing to do with 'bubbles', or wing height, or footprint, or brightness/dullness of the shuck, or any proclaimed 'trigger'. Think not? Get a bunch of old - even used flies(but take any tippet off the hook). Toss them in a water where you know there are trout and see what happens.