Do you know where they were fighting or their nationality?

The only guess I could come up with is if they were somewhere in the Pacific. Weather conditions were known to be pretty tough, maybe it was more comfortable?

The uniforms the Germans wore were woolen and become extremely hot during physical labor and in the warm/hot months. The footage you saw was likely taken in the summer and repeatedly carrying heavy shells in the summer heat, wearing a heavy wool uniform, would become extremely hot and uncomfortable.