Teleporter exits destroy themselfs if something is inside the teleportation area of the exit.
This is to avoid exploiting or getting people stuck.
Rarely this can happen, even if nothing is visually inside the tele area.
Mostly due to poor map design.
I guess you just got really unlucky.
The only way to avoid this, is by just checking and memorizing wich places don't work / are safe to build teleporters.