Have just had this very problem and this is the only thing I've found on it after searching, so 2 years and a patch or two later it still seems to happen, this being BI 1.6

My small Brutii force attacked and annihilated a Macedonian phalanx then I sent them to join my larger reinforcements before over 2000 irate Macs got to them. Everyone made it except my generals unit, who seemed to be recreating Glastonbury Festival in the middle of the plain. Their icon had crossed swords on it, although the enemy were all deceased, and the tool tip revealed they were "Celebrating". The enemy archers and pelasts opened up and only 10 out of 49 made it back to the safety of their own lines, the party having lasted well over a minute despite repeated right clicking.

What to do?