Well...
What level was the turf they played on? It is important to keep upgrading, as better turf lowers injury rates.

Were they very tired (two or three match day)? Did you train them a lot? They should be near to full condition at match start.

Was it ordered hard attacking and tackling? The opponent may also have set to hard/hard. This one gets me, I rarely set it to that, but when I do, half the time I get injury.. and my team rarely get injuries.