Try making a burrow check by putting a location on the lurker and every second or so making a dark templar owned by a player you cant see at the lurker.
If the dark templar appears at the location, the lurker didnt push him out of the way so the lurker is burrowed.
If the dark templar does NOT appear at the location, the lurker pushed him out of the way.
In that case, create a burrowed lurker at the location and remove the unburrowed one.
By the way, use death counters so you don't need a constant wait trigger conflicting up the place, but you should probly know that already