The above answer is correct. The reason you don't see this with the tapes is that they don't go below 20HZ. You are seeing arm/cartridge resonance because the effective mass of your arm is too high for the compliance of your cartridge. This sub-sonic "signal" is actually using up a lot of your amplifier's power reserve. You could reduce this by buying a lower compliance cartridge or, if it doesn't bother you and you don't notice it, just live with it.
Woofer pumping - what gives?
Some years ago I noticed that regardless of the speakers, cartridge or preamp in the circuit my woofers pump wildly out of control the more I advance the volume while playing an LP. They don't seem to be responding to the musical signal but something else. The sound is fine. My humble rig is a Dual 1219 with a NOS Shure M91ED cartridge. Tapes made from the same LP's do not exhibit this odd behavior. Who knows why woofers look like they're going nuts while playing an LP?