I have a magnum 350 canister that does the samething once in a while.
Believe it or not you have an air leak at one of your connections, even though you don't see a leak when you turn it off. On my canister it is usually the the hose that comes from the tank and the leak is one the the two o-rings in the quick disconnect. Hope that helps.
Nobody wants a broken fishtank, capish?