First of all, the Memphis players running towards the BYU players/sideline to taunt and instigate was intended to incite the fight.

Second, Koroma was pushed from behind into 97 (notice how Koroma spins around immediately after the contact with 97 to see who pushed him), and then no fewer than four Memphis players start punching Koroma. Also, notice 68 jumping in and trying to swing his helmet, though ineffectively.

There's no question that the fight was started by Memphis; their own coach knows it based on his comments immediately following the game. He said they need to learn how to deal with success (i.e., don't taunt the opponent and incite a fight after a hard-fought win).

It's unfortunate that BYU was dragged into a fight like this, but everything I saw (after reviewing the tape many times) suggests that this was a Memphis-instigated fight.