I suspect that they were very near the surface and the shadow spooked them. I've seen it happen when a great blue heron flies over as well.

I've also noticed bass following along behind a night heron as he stalked the shore. When the heron spooked the bluegill, the bass would swoop in and grab a meal as the fish darted away from the heron. The next time I went out, the heron was following the bass as they cruised the bank. When the bass went after fish and they darted towards shore, the heron was there to grab them. Seems like they've develped a symbiotic relationship at the expense of the bluegill.

Jim Smith