I witnessed something that peeked my curiosity. I saw hundreds of ants moving in a straight line on a curb for about 50 yards. 1. Why would they do this? 2. 99% were moving in the same direction but occasionally one would fight the tide, why?
The behavior you witnessed could have been a colony moving to a new nest site. Parts of an ant colony or an entire colony move to a new nest if the previous has gotten too small or if resources around the nest have been exploited. It's also possible that the ants were performing a "raid", where they quickly swarm a food location. Did you happen to observe whether the ant workers were either carrying brood items or prey items? This might help to understand whether they were moving to a new nest site or were on a raid. The solitary ants moving against the tide were likely recruiting those which were still behind.
All the best,
Arista Tischner & the AntAsk Team