This is normal for a phono stage with the old 1953 RIAA curve. The update, in 1976 included a sub-sonic roll-off to avoid this sort of thing.
IMO it is all bad. It is your arm/cartridge being excited by structure borne vibration as you walk about. All output below about 2x the arm cartridge resonance is either spurious or inaccurate anyway so a phono stage with a 1976 RIAA curve or a high pass filter will fix it.
Putting the TT on a wall shelf or proper isolation will reduce the excitation, and is a good thing to do anyway, but will not fix the fundamental problem, which is that the non-accurate part of the cartridge output is being excessively amplified.
Edit: Certainly having a compliant cartridge in a heavy arm can move the arm/cartridge resonance down to a region where it may be excited by warps. Having a low compliance cartridge in a low mass arm will move the natural frequency up to a level where it may start to be excessively excited by actual music (though this is not cone flap) ruining the phase and amplitude accuracy of the bass.
A good match between cartridge and arm is very important, but it will not prevent cone flap whereas a high pass filter will.