I've been slowly getting to the bottom of what I think the game is doing when it transitions from the ragdoll to keframe state. It appears Bip01 Spine01 (and likely Bip01 Spine1) factor in the game acuiring a new position for the keyframe system to re-start from (Bip01). The game also uses it to determine which animation to play (the one closet matching the actors orientation) when getting back up, it's limited to 2.
As for your question, the flopping noise you hear is the game re-alinging the actor to match the orientation relative to the Bip01 Spine01 bone and it can get ugly with rapid twisting of bones. I have a video of one of my new custom creatures which at full speed you hear the noise and notice some slight movement but it's not until you slow it down that you see some horrendous twisting of bones. I'm not sure what rotational controllers the game is using when it does this but if it works on Euler rotation then it might also be due to gimbal lock.
The get back up animations are special idle animations and are defined under game play so that they play automatically when certain conditions are met. For whatever reason the game has failed (possibly a timing issue) on that branch of the game play idles and has defaulted to a simply dumping the actor back into the mtidle (base idle).
@KiCho666, Geck Power up will prevent it from crashing when you access the ragdoll settings but I've had little joy with that as it appears the Geck is failing at loading up controllers (most likely the reason for the default geck to crash).