What are the main contributing factors that cause turnovers? 

In no particular order...IQ of the offensive player, Ath/Spd differences between defenders and off player, BH/Pass ratings of the offensive player, Def rating of the defense, type of defense run...fatigue probably plays a role in here as well, but I'm not sure if that'd be independent or just as a product of how fatigue modifies other things mentioned already. 

ETA -- IQ of the defensive player too (can't believe I forgot that in my original list...

and shots taken....guys with very low distro turn the ball over less..

Defense of the opposing players/teams is a big factor in my experience - it's one reason why having good defenders is so critical to HD success.