I think I figured out what happened. Friday night I did a small water change. Also, when I do a water change, I un-plug the ATO and plug the new water pump into the ATO socket. This way the water will replenished as I drain the old water. Any way, I woke up this morning to the sound of the return pump sucking air from the sump. Turns out that I forgot to re-plug the ATO. SO, I think the combination of a water change and rising salinity triggered the spawn.