OK, so I found a leak and wanted to turn off the pumps in the hope of fixing it. I did and then a minute later....FLOOOD!
I turned the pumps back on but...my return pump wouldn't fire up. I panicked and worried my whole tank was going to spill out on the floor. I called KPotter2 and he talked me down as the water stopped flowing out of the tank. Thankfully.
I thought that maybe after putting in rock I'd gotten in too much water volume, but that couldn't have been it. I didn't put in that much rock. I'd tested the backflow before I started the tank and all was fine. What the heck?
I siphoned out the water to the previous pump off fill line and finally got my pump started again and when the water returned to the tank, I didn't even have enough water in the sump to run my skimmer. I dumped back in what I took out and all was fine.
But what caused the overflow?
Then it hit me: Over the weekend I also extended my returns so they went into the water. I had one of them way down in the water to try and direct flow toward on place. It was extended just to the level where the water had drained when the pumps turned of.
Answer to the mystery: Backsiphon...