Herbicide drift is a real possibility. Over on the Garden Forums, we often see examples of this terrible consequence of herbicide use.
There are other possibilities...one that comes to mind is pond turnover. Did you happen to notice that your pond water suddenly turned cloudy, muddy looking? When it happens dissolved oxygen levels fall below that needed to sustain life and just about everything dies suddenly. This is the time of year when it is most common. How deep is your pond? If very shallow say two to three feet, turnover is not the likely cause.
Another somewhat sinister possibility is someone put rotenone in there. It is a non-selective killer of everything in the water. Some pond managers use it when the pond has reached levels that require complete start over.
Another possibility...have you or anyone else recently added hay to your pond for management purposes. Today hay can contain residual herbicide which will kill everything in the pond.
Of these possibilities and considering the time of year, I tend to suspect turn over as most likely.