When there is a void or the vent pipe gets lifted up by the frost.

This will let mud, stones and sediments into the trap.

Here is a picture of a trap the broke you can see the inlet from

the house and the exit of the pipe. Mud kept falling into the trap

because the risor pipe was no longer attached.

this second pictures is the repair. Two pipes the outside pipe is for the frost to grab unto and the inside pipe is the vent.