I have googled "BEHAVIORAL DRIFT" for those that care.
Essentially mayflies and many other aquatic insects are prone to swimming up from the stream bottom (often in masse) and allow the current to sweep them downstream... perhaps to areas with better feeding conditions or to move to an area better suited to true emergence. During this time these "swiming nymphs" are quite vulnerable to feeding trout that find them an easy meal.
Over the years I have noticed that in the weeks prior to various major hatches the use of nymphs that suggest behavior drift work like gangbusters. This past weekend I never tied on a dry or used a dry dropper rig but went immediately to a BHPT 34 inches under a palsa and didn't not stray from the strategy all day.
You might do further googling to better understand the phenomena.