In any case my wii usually freezes when it needs to calculate lots of data (i think).
For instance it was running just fine. Even hosting i didn't get much lag.
Now in the tenth game or so a hacker comes along, he starts spraying explosions, resulting in a lag increase.
Then another dude joins, and i'm thinking he had a crappy connection cause from that moment on the game's lag increased to the point that i got a new screen every second or so. then the wii freezes.
The other times it froze was also when i'm thinking the wii was receiving/reading new data. Like at game ending, someone new joining, getting exp from a killing spree, lots of explosions in the game, that sort of stuff.
So basicaly i think my wii freezes when it's just unable to properly process all the data.