After seeing Alekan discuss it here: http://positech.co.uk/forums/phpBB3/viewtopic.php?f=43&t=11155#p72590
I am also frequently observing particular unit of mine holding fire. In this case they are using fast missile launchers, and are easily in range of a large number of enemy vessels that they have orders to attack.
It is difficult to tell, but gunners may be selecting units that are slightly out of range. On it’s own, this is infrequent at the most.
However, the effect is exacerbated tenfold with vulture and cooperative orders. Removing these orders can turn close matches into dramatic routs - something is up.