Newb question; are braking bumps only formed by locking up the rear wheel (skidz) or do they form even if people aren't locking up their brakes?
My guess is they form anyway. If you are braking then there's a force on the ground from the tyre. When you hit a bump the wheel gets kicked up a bit (even if it doens't leave the ground altogether) so the force is less, then the wheel comes back down and the force is more, so a dip forms after the bump. Then the far side of that dip becomes another bump, so the wheel gets kicked up again and so on and so on, the bumps spread down the trail.
The ground has to be fairly firm for this to happen I think, because otherwise the wheel will tend more to plough through the bump instead of being kicked up by it. Also, suspension changes the dynamic because the wheel goes up and down a lot more. So the bumps get bigger I suspect. That would suggest that the more travel, the larger the bumps and the further apart they are, which is kind of what you see on DH courses. There, the bumps are a lot bigger and more widely spaced than the ones you see opening up on say a 24 hour course where most people are on XC bikes.