Most of the new stuff can vary the stall speed of the converter by controlling how much fluid the converter gets (through the computer). Some even have variable pitch stators (like the old switch pitch TH400's, but more than just 2 positions). Or so I hear. It could be a quirk of the computer, thinking you were stuck or something, trying to give you max torque.
How did it 'go' when you stood on it
without the brakes on?