Chatter bumps are caused by the suspension of vehicles with a very loose compression and rebound area at ride height. it can be caused by rails, sxs's, trucks, and just about anything in the dunes.
Essentially, its wheel hop/axle wrap while under power. the suspension is loose enough that instead of a steady apply of power to the ground, teh light rebound allows tire to drop as it digs, vehicle weight catches up, traction is caught, tire grabs and suspension is slightly compressed from light compression, tire looses traction again and the process starts all over. that is an overly dramatic way of putting it the increase and decrease of traction is only slight and the suspension movement is only slight but its enough to cause some chatter. Now you run the next guy over it and the next guy and so on with ALL loose rebound and compression and the chatter pumps are greatly increased because the suspension of the car is eating the bumps up, loosing traction, gaining it, moving sand back to those bumps and increasing their size. Same reason whoops are formed.
SxS's have a heavier play in this roll as there are FAR more of them and the internal bypass shocks they run are setup to be nice and squishy while at ride height. they will not feel them till the chatter size reaches a point that the interal piston is now moving behind the first bypass or the springs are hitting the secondary stop.
desert races have had these chatter bumps for YEARS due to teh high travel numbes of trucks, buggies, and other cars. specifically in the braking area for a corner and accelerating out of a corner.
the only way to stop it is to stiffen up the high speed side of valving but that makes for a poor ride over ruts, small rocks, and other things that can induce a hard ride. the only time chatter is created is on soft soil like sand.
