The only reason for the unit to end up in the middle would be if you moved the location to a unit that didn't exist.
For example, if you had moved "Morph" to a Marine at "Anywhere", but there wasn't actually a Marine on the map, the location would go to the center of "Anywhere" which also happens to be the center of the map.
QUOTE(Jammed)
What was suggested: there were suggestions like "first move the marine to the location of the zergling, and then remove the zergling from the battle arena". That was unacceptable for me, because it would Morph not in the same spot.
That's exactly what the triggers you posted do, except with the added action "move all men owned by current player at "Battle Arena" to "Morph"" which makes sure the Marine is moved to the same spot the Zergling was in.
There's a topic about a similar problem in the Assistance forum.
They were basically trying to replace a Marine with a powerup.
You can
read the solution I posted if you want.