1. Sump overflowing because your return line siphons water from the tank, or if your water levels is not set correctly in the sump to accept water from tank in an internal overflow.
2. If HOB
overflow - incorrect setting of water levels inside the boxes so you lose siphon with a power out & tank overflows when power comes back on.
2. If sump is level or above tank, (some kind of reverse sump system, like in a pond), then water from sump can overflow tank from the drain or return line.
Article on how to setup the water levels so you don't have a flood (also note the link at bottom to a 3 part article on setting up a sump.):
Melevsreef.com - What is a Sump?