This was a little while ago Gerry, and while I only have DC Barbarossa so far, I've seen some play throughs on TY etc, and I just want to point out the that image you have posted above in OP, is an actual unit itself of primarily Artillery, not a 'stack'.
It is a unit made up of its constituent parts combined together of Infantry, Trucks and different pieces of Artillery, 4x 105mm & 1x 105mm and a bit of Flak, 1x each of 88mm & 20mm. This is a unit together, not a stack of different units. You can't separate these different components.
I'm not personally sure if the 2x Infantry represent a security element of some kind or are there to represent the gunners themselves with their own self-protecting rifles or whatever, but yeah Gerry your initial inclination that they are present for protecting the guns is right, I'd say.
Hopefully you figure it out or have figured it all out by now, cheers.