Why does my scene crash when my characters starts to physicalize?

You have a very large scene with many entities in it and you would like to physicalize all of them but your scene keeps crashing every time you press play. Check your script editor, you should have this message popping.

When exceeded the number 65535 rigid bodies in your scene please pay extra attention to the Physics environment as your ground is maybe colliding with every entities

The workaround would consist in splitting this big Rigid Body into multiple geometries and to convert it into separate Crowd Rigid Bodies. The physical collisions will be much easier to compute. Also don't forget to put your Physics Locator Environment Mode to none.