An army of one hundred thousand is a long column on the march. The ones arrayed on the high ground were just the first to arrive; tens of thousands more were still on their way.
The defeated remnants fled south for their lives, while the oblivious legions behind them continued to march north. The battlefield was waiting for their support. Even if they couldn't get there in time to help or claim any glory, they at least had to make it for the plundering of the city. They needed to get something out of it, or being conscripted would have been a total waste. So, they were in a hurry too.
Two groups, both in a rush, collided head-on. They came from different territories, served different lords, carried different banners, followed different commands, and even had different accents. In the heat of the moment, some weren't even speaking the common tongue, blurting out words in their regional dialects instead.
