It is possible that the holes or at least sections of them were formed prior to the slab winding up in its current position and orientation. The bottom of the slab appears to have sheared off from another layer. The holes and/or sections of them may have actually been formed when the slab was in a more vertical orientation at some point in the past. How many times it may have shifted and to what angles/orientation over the ages, eroding away all the while into different shapes as water eroded it at different angles before winding up in its current somewhat horizontal orientation and position is anyone's guess, but looking at the surrounding terrain, it looks like it likely may have been at an inclined angle on the hillside prior to shearing off.