The ants entire work structure was just destroyed off of one rock, it's like a mountain to them
They operate by scent even more than sight. Each one is following the trail left by the leaders. If the rock had been there at the start, they would have gone around or over with no problem, but the "map" has been changed on them, and the scent-trail now leads them straight into a "wall", totally confusing them. "He went through here, I know it!" "How is that possible?" "I DON'T KNOW! I JUST KNOW HE WENT THROUGH HERE!"