League Boomer here:
Gather round and let me tell you a tale.
Before the cannon was added, almost every player would take teleport as a summoner spell, because it was on such a short cooldown. In fact the cooldown was so short that you could instantly teleport to a lane at the start of the game, and then have it up again by the time minions spawned.
So one player would teleport to top tower, run forward into the bush (this is pre alcove, and pre 3 bushes) and drop a ward for all the other players to teleport to (there wasn’t a restriction on what you could teleport to pre ten minutes).
If the other team wasn’t doing the same thing, you would get free kills on whoever came up there, and then when minions spawned everyone would reset and teleport to their respective lanes with an item advantage.
It started spreading, because you had to match it or you would always be starting lane phase from behind. This is why they disabled teleport in URF, and why the cannon got added.
As for why top lane? Well it was the least expected place to find 5 people level 1. There have always been cheese strategies where people camp a bush in bot lane level one (think kill lanes like kha’zix lee sin), so bot laners were more likely to ward the bush and potentially escape. Mid lane had no easy bushes to ambush from.
But top lane? Oh man, top lane players were classically afk pathing to lane, while picking their nose or alt tabbed. In normal games, only teemo, Sion, and Darius would try to level one cheese. Because there were only two bushes, both pretty big, as long as you got into your bush before the opponent could get past the gap into your bush, you wouldn’t lose the level 1 skirmish.
It was like taking candy from a baby. X top laner would walk up, not knowing they were about to be the star of a 5 on 1 gangbang scene for Dogfart, and it all snowballed from there. Soon everyone was taking teleport and matching the play.
Story time over. Go back home I don’t have any extra sleeping space here. Billy! Throw some water on the fire I’m ready for bed.