Here is my 60 gal setup.....This is the same tank that i had my fist and second spawn........ITs wierd how they all stay on the left under the slate tiles but when they spawn only two go to the right and into the cave???? In the second picture you can see that one male is fighting others off from this cervice in the slates but there is never breeding in there??
Perhaps the zebras gather on the left side of the tank due to them favouring the cover that the slate provides. Another thought, does food collect around this area? I suspect that it might because of the position of your powerhead.