Probably a turnover. It may be too late by now, but if they haven't already kicked the bucket in large numbers, driving around the pond in a boat with an outboard motor will oxygenate the water and prevent a fish kill. If I were you I would go to the pond NOW and check to see if fish are dying. If they're not dead yet, or just a few are dead with lots of others piping, you can still save the ones that aren't dead but you'll need to get an outboard in there pronto.
Another way to do it is to back a bushhog into the water a few feet and run it full-bore for a few hours. I haven't personally used this method but got it from people who have either done it or seen it done.
Several years ago I happened out one afternoon to a pond I was working with to find bluegill by the dozens piping at the surface; a few were already dead. We had had several days of cloudy weather in a row, and the pond had a very good (probably a little too good) plankton bloom, so probably the plankton had crashed. I rushed back into town and got my grandfather's V-bottom with a forty-horsepower Johnson and sped back out to the lake and proceeded to circle it for a couple hours. It worked; they stopped piping and only a handful died, whereas probably a large percentage of the population would have if I hadn't happened out there at the right time.
