unexplained

adj

1. (not-comparable)Not explained; of unknown cause or origin.

   “Why this already very fast train should be speeded up still further, when none of the other more easily timed S.R. West of England trains has a single minute pared from its schedule, is unexplained - unless this is a playful dig at the Western Region, most of whose expresses, by reason of additional stops, will be decelerated from the same date.”