Subwoofer "Thumping"

When listening to vinyl, at somewhat high volumes, my subwoofer starts to vibrate rather loudly. This only happens when listening to vinyl. What causes this?

REL Q-150E Sub
Parasound PP100 phono pre

You may also be suffering from acoustic feedback, i.e., the sub is causing the turntable to vibrate, which in turn is picked up by the cartridge, amplified, and played back through the sub for even more vibration. You can test this by pressing a finger on your turntable plinth during play and observing whether this changes or stops the sub's oscillations. If so, tweaking your turntable's suspension may provide a solution.
My thought is that the arm/cartridge has a resonance mismatch and the woofer being able to respond to those low frequencies, is only doing it's job. What kind of arm/cartridge do you have - I suspect an arm with high mass, and a cartridge with lots of suspension travel.