LostSoul
Adventurer
That doesn't explain why you can't just leave your opponent on the other side when the portal closes. Or why you can't stay in the feywild. Sometimes being trapped there may be a better option than teleporting a few squares.
Because the portal has not been opened "well enough" to allow you to stay on the other side. The creature will pop back.
It probably has something to do with magical thermodynamics.