If I remember correctly, the classic Sikorsky Blue Book shows ground resonance happening because of lead-lag making the effective center of mass move off the rotor shaft centerline. That puts a moment on the shaft which makes the aircraft move and the gear responds to that movement. I've also read a bunch of Ray Prouty's work. The teetering rotor is locked rigidly with the blades 180 degrees apart so they don't have the same issue as the articulated rotor. That's also what I was taught in groundschool.