6pm MK closings have tended to mean a special event of some sort. For example, Grainger had a corporate event 2/12/18 and the park closed at 6pm. The fall Gartner IT convention is 10/20-10/24 (tends to rank as one of the bigger/biggest WDW conventions each year), maybe something going on there (that is total speculation on my part).
There’s usually a CM anniversary party sometime later in the year, although those have been 4:30pm closings. Dates were 11/5/2018, 11/30/2017, 11/16/2016, so both the time and month don’t quite fit.