I hate to be the one to teach people how to take advantage of a bug but, as I understand it, this is related to both lag and game mechanics.
As others have already mentioned, there's a normal amount of lag between the time an enemy spawns in and the time that they materialize on your screen. This is normal Internet lag but it only accounts for fractions of a second.
I've been led to believe that the game updates players nearer to you faster/more often than those further away so as you spawn out of the camped field tower you've been sitting in the materialization delay is at it's shortest. If, on the other hand, you go to a field across the map from the one being camped the updates to you from the other side of the map are at their slowest. Therefore, if you start from a distant field, go to the tower of the field you want to spawn from then immediately spawn you maximize the time it takes for the camper to get an update and for you to materialize in front of them. This is what allows the spawner to get the turret around before you see them.
While this is an exploitable bug, at least if everyone knows how it works the field is even and because of this camping hangers is not safe.