It can be of the following reasons.
Poor back pass - not being aware of where the opposition is, not concentrated enough
Poorly anticipating a move, running into an offside that easily could have been avoided
Sliding in a tackle incorrectly, getting ghost passed.
Coming out to a cross totally wrong ( goalkeeper)
Scoring an owngoal
A defender not being aware of his teammates, making my offside trap fail.
Missing an easy pass going straight into the opponents feet.
Causing a penalty.
etc etc
Ok, in those cases I could see 1.5 errors per player per game.
Would be beneficial if we had errors leading to a goal scoring opportunity stat?