Formations to prevent unit backtracking?

I sneaked about 25 hand cannoneers behind enemy lines, set on Stand Ground, to knock out their trade. Since I was mostly using Paladins, they had a horde of Halberdiers to defend but didn’t expect the HCs.

With those two units, I had horrendous LOS vision. The Korean player had 8+2 range on his Siege Onager and popped up with almost no warning. I tried sending cavalry to find him in the fog and sent my HCs running. But as they did, half of them filed backwards, right into his fire, to regroup before running!

It was carnage! He said it felt good doing that then quit since it was game over by then.

My question - should I have tried splitting after telling them to run? Does that interrupt the regrouping? Normally I don’t mind, but against onagers, you can’t afford to file backwards into fire.