Fact is no-one outside of the club knows what the actual circumstances underlying this are. It may have been work-related, a personal issue, petulance, naivity, poor counsel, or none of the above.
We'll probably never know unless Duk himself provides any further detail, as refreshingly the club have handled this with a degree of restraint and professionalism throughout and I doubt that will change.
Unfortunately for Duk though, that's unlikely to extend to the fans, so between now and January he's going to have to put a right shift in to demonstrate he deserves minutes in the team, else the taxis will start closing in.