+1. It's best to turn off your skimmer when you are doing tank maintenance. My power once cut off for a split second, and I thought it was no big deal. Went back into my sump closet 30 minutes later to an empty top-off container, and an overflowing skimmer. They are pretty consistent with skimming the tank under normal conditions, but very sensitive to changes. Especially water level changes. I've learned not to have my skimmer cup emptying into a gallon by making this very mistake...
Best of luck!!!
Edit: Beware that some tank chemicals will cause the skimmer to do the same thing. Too much coral putty and red-slime remover are a couple of examples...