They weren't hitting the ground, they and the MDUs were on level ground, and both MDUs were very close together. I had a swarm of tanks and infantry all shooting at them. I started off by telling them to shoot at the closer MDU, but it wasn't taking any damage, then the one behind it randomly exploded (I think my guys were hitting it instead). Then they started shooting at the one I told them too. About half the shots were hitting and half were missing, but no damage was done (until I bombed them!). I'll have to fix the victory-relevant thing in my mod...