The system is responding to subsonic frequencies. These can be generated by warps in records, turntable rumble, or mismatches in tonearm/cartridge combinations. Or it is possible that the system is not capable of reproducing certain low bass frequencies, and is unloading the bass-reflex cabinet, causing the woofer to bounce because the air mass in the cabinet is unloading. Or it could be acoustic vibrations coming up the stand,into your turntable Or it could be a combination of all of these things.
If you notice that it only happens on certain records, and not on others, it is probably a warped or defective record. If it happens more when you play loud, it is probably acoustic vibration entering the TT.
I'd try moving the turntable into a different spot in the room, where there may be less bass frequency buildup. If the TT is behind the speaker, especially in a corner, this is bad placement.
If you notice that it only happens on certain records, and not on others, it is probably a warped or defective record. If it happens more when you play loud, it is probably acoustic vibration entering the TT.
I'd try moving the turntable into a different spot in the room, where there may be less bass frequency buildup. If the TT is behind the speaker, especially in a corner, this is bad placement.