Croxton :
As far as I'm aware you can only get injuries in league or tournament matches
The biggest factors are toughness, MB and rhythm. Low toughness vs high strength equates to potential injury, and both MB and too high rhythm cause you to get injuries far more frequently. As a rule, I can tell when I face an MB guy in league or tourney as its almost a guarantee I injure the maximum 2 guys of theirs in the match
If you're getting injuries, but not using MB, then look at your toughness. If that is quite high (compare it to the strength stat of the opposition players) then it's purely down to having rhythm too high for your guys
How can match bonus cause players to get injuries, you can only use bonus in map games not in tournaments and championships, but you said injuries is only possible at tournaments and champs. so if you injured players from opposition teams, match bonus was definitely not the reason they got injured