What causes these big gathering of ants on a sidewalk?
Here is a whole bunch of ants gathered on a sidewalk around dusk, I have seen this before so its not totally uncommon.
What would lead to ants doing this?
1 answer
These look like the pavement ant (Tetramorium caespitum)—also known as the sugar ant in parts of North America
The are very common in the US, and the photo indicates the are probably having an ant battle.
Although you may see pavement ants during the day, they are most active at night. If you happen to have more than one colony on your property or another one nearby in the neighborhood—and you happen to be observing at the right time—you just might witness the unique event of an ant battle or its aftermath. Pavement ant colonies will fight for territory, and these battles can leave hundreds of dead ants on the battlefield. Source
Your image look much like this one on Wikipedia 
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