Changing barometer as Rick says. Not always low, but a falling barometer. A good place to look is the MHL website. If you look at the current barometric pressures its about 1020, and has been for the last few days. But, there are always incremental changes over small periods.
The other thing to consider is rain, it has a huge influence on where the fish will be. Both in the system and in local areas. Once there is a bit of rain at this time of year, its a trigger for the fish to move en masse. Sometimes they will move downstream in preparation of spwning, sometimes they move out of the main body and into tiny creeks to feed.
Matt