Weird, I've never heard of something like that happening before. I'd rule out anything related to backports, that would require having been inside the castle a bit earlier. Spirit Jump also doesn't fit, it only works on friendlies. Rune Draw is capable of pulling an enemy close (including enemy players), except that to my knowledge it requires line of sight, which is blocked by the gate.
There's a not that uncommon modification that negates the line of sight issue for the caster. In theory this would explain perfectly what happened, except that the champion would have to be really braindead to use this several times in a known recorded session, for no obvious benefit. So, ruling out foul play, let's try to get some more data to work on:
- Did only one player get ported in at a time or several at once?
- Was it like walking through the gate or a sudden jump over some distance?
- Any chance you could have received a popup that was confirmed automatically?