A magnetron is a special type of vacuum tube and can suffer from the same failure modes as most vacuum tubes. It could be arcing internally, the vacuum seal could have failed letting some air into it, it can develop shorts to the filament, etc. All could cause the magnetron to draw lots of power without actually putting out much if any microwave energy. Additionally, if the magnets on the magnetron were damaged it could cause something like this as well as it wouldn't oscillate like it's supposed to.
Of course, first start by measuring for correct high voltage to the magnetron (~2000V typical, high voltage probe a must!!!). If this seems ok, then magnetron is faulty and will need to be replaced.
Also if it is a traditional transformer + voltage doubler capacitor setup (as opposed to modern high frequency inverter), check the cap.