At what minute of the game did this happen? I noticed 1 Crab sitting on a Grizzly wreckage at about 39mins that seemed out of position, but it still fired at enemy units.
However, I suspect it had something to do with putting the Crab on retreat with a retreat zone west of your spider factory. Retreating units must be individually selected and told to move (unless you circle no non-retreating units) - they drop out of control groups and will not be selected when you circle units and give group commands, until fully repaired (this can be changed in settings under selection priority).
