Well depending on which rumour you want to believe, I guess adds weight to the fact that Harris' sacking wasn't necessarily planned. I'll let you decide from the below as to what we think may have caused his abrupt departure:
RUMOUR ONE: Following a usual Thursday staff meeting, Harris was called in to see Scally & Jackett to discuss matters with regards to his assistant manager David Livermore. It is believed that the club hierarchy were still incredibly unhappy about Livermore's actions after the Crawley game in which he allegedly threw a punch at a Crawley player, for which the club received a fine from the FA and also Livermore himself suffered a separate fine and four game touchline ban. That ban ended for him to return to the dugout for the Crewe game, which we subsequently lost. Now I wasn't at the game, but if Livermore got particularly animated during the match, more alarm bells may have been raised that he hadn't learned his lesson, and with the club having only recent delivered a pitch to the fans to improve behaviour, having a potential "loose canon" amongst the ranks does little to help promote that fact. The rumour is that in the meeting, Harris was told to relieve Livermore of his duties and to either appoint a new assistant manager that was lined up or at least look elsewhere. Harris supposedly refused to sack his mate and a disagreement occurred and Harris lost his job in said meeting (with BG's approval).
RUMOUR TWO: It was Kenny Jackett that got Harris sacked and not Scally, apparently Scally was actually against his sacking and felt that with the club's position in the league we should endeavour with NH. KJ and NH apparently disagreed multiple times in the summer over signings of potential strikers. After the Colchester defeat, KJ wanted NH to leave there and then but BG felt he should be put "under review" and after the loss against Grimsby, his card was marked. The general feeling from KJ was that NH wasn't maximising the squad to its full potential and by the time of his sacking, all of the upper-management with exception of Scally were in agreement for his departure so Neil was relieved of his duties. This rumour has also has had fuel added to the flames by a tweet made by NH's wife suggesting that someone was a Snake to Neil, considering Neil & KJ had worked together before and Neil had often leant on KJ in the past for advice. Further weight is given to this rumour/theory from the fact we saw a sudden change to the style of play from the Doncaster loss onwards where its believed NH was changing the style in a bid to protect & save his job.