running uphill like it's downhill

Apologies if it's been covered already, but i did a quick search and nothing appeared to mention this. Had a large battle, Julii (me) vs Brutii. I set up on one high hill/mountain. extremely steep angle, i'd guess around 45 degrees and the brutii look like ants as they're approaching. Good pickings for my archers and onagers, pound away on those poor souls as they approach my perch. Getting closer and closer, and suddenly i realize that brutii cohorts are flying up the steep approach like they're skiing downhill. In a slightly herky-jerky fashion, it seemed like they would kind of shuffle about, then squirt up the hill, running as fast as they would on flat ground. Wasn't my machine having a problem, as the rest of the units were moving normally.