Fork 'pump down' explained
Pump down, or packing, occurs when you hit a series of bumps after dialling in too much rebound damping.
Because you've over-slowed the rate at which the suspension can return once it's compressed, it's still returning to its starting point when you hit the next bump, forcing it to compress even further.
Rebound slowly starts once more, from an even more compressed state, before hitting another bump, compressing still further, until you use up all your suspension travel.
If the problem manifests itself at the forks in a corner, it's not unusual to find the front tyre "washing out". You then crash.
If it happens at the rear, there's usually an accompanying loss of grip from the back tyre , again a potential recipe for a get-off.