Haven't noticed a rhyme or reason so can't reproduce.  I have noticed it both with military units (single or multiple selected) and engineers.  I have also noticed at least with engineers it can happen even if I queue orders.  With military units I tend not to queue orders.  The blue lines appear showing destination, with engineers queued buildings will appear where I place them but the units never move.  I have only played a handful of games but when this has happened I have gone on to other tasks and only noticed the problem after circling back.