Tanks in apartments?

[PeeJ]

I have had an overflow happen, and I live in an upstairs apartment. Luckily nothing happened downstairs, but it was caused by my maxijet that's hooked to my uv sterilizer. The power went out, and the pump kept siphoning out the water. I wasn't home, and lost half of the tank water. Like I said though, I was lucky because their was no leak downstairs. I moved the maxijet into the sump to keep that problem from happening again. I did get lucky, and I run a 75g tank. Upgrading to 100g.

1) Why did it siphon water out of the tank? Was it pumping out of the tank, through the UV and into the sump or something?
2) I'm shocked that 30 gallons of water on the floor didn't cause major damage.