It's basically random. You can think of the matchmaking database as a huge SQL table (with each player having one row / entry in that table), and whenever someone wants to play online, they'e like SELECT * from mariokartwii where hoststate=2 and suspend=0 and rk='vs' - or translated to english - get me all players that are currently "host", that are not suspended (for example, when you're currently on the track selection, a room is "closed" for new players), that are playing worldwide races.

Wiimmfi then returns a list of up to 6 players where these conditions match - so, up to 6 players that are currently hosting a WW room - and the game randomly picks one and connects to them.
Wiimmfi itself, while returning these 6 players, will take stuff into account like how stable a room currently is - so if there's multiple players with a high "conn fail" value (see wiimmfi.de/stat/mkwx ) then Wiimmfi will not return this room to players searching for a new match, as that'd have a very high chance of killing the room completely.
So if the players in a room have a shitty connection and a high "conn fail" value, Wiimmfi will "assign" way less new players to that room, to prevent it from crashing.

And if the game failed to connect to the existing room it's chosen, it switches around to "host mode", creates its own "entry" in the SQL table, and is basically like "Okay, I am now hosting a WW for a couple seconds, let's see if someone else is also searching at this very moment and finds my room" - that'd be how your 2-player match most likely started.