Doesn’t look like an implosion.
The mesh would likely not remain as steady as it does…
Have you tested the PHAT asset of the specific model out without gravity?
The movement could be caused by a single leg (probably on the right side based on video) “jumping” to a snap when physics is turned on.
You’d see it less on smaller models, but you’d still see it.
To debug these things, turn on Slowmo at .5, enable anim debug. Record the screen, and then analyze the footage.
Ps: it looks like the mesh is suddenly upside down when ragdoll goes on?
Review the footage in slomow.