Check the pedal spikes. If they are sticking out it might be making a pivot point for the pedal. The weight of the kick keeps the heel plate off the ground until you step on the pedal. Does the drum rock up and down slightly as well?
To echo GruntersDad, in my experience every time this happened to me on an otherwise level floor it was because the front of the kick drum was raised too high. It's counterintuitive, I know... but adjust those spurs and see if it helps.