Perhaps something like a two triggers. A trigger (1st) that removes the unit, and a trigger (2nd) that removes the spawned collapsed rocks. 2nd starts off and is turned on right before whenever you're about to remove rocks. Have the 2nd trigger turn itself off.
2nd trigger could probably just be something like
Unit enters entire map
type of triggering unit = collapsed rocks
remove triggering unit
turn current trigger off
Perhaps something like a two triggers. A trigger (1st) that removes the unit, and a trigger (2nd) that removes the spawned collapsed rocks. 2nd starts off and is turned on right before whenever you're about to remove rocks. Have the 2nd trigger turn itself off.
2nd trigger could probably just be something like
Unit enters entire map
type of triggering unit = collapsed rocks
remove triggering unit
turn current trigger off