Hi Elayna,
Well, I don't know that answer for sure, but I have a few ideas. 

First, the mass eruptions of pavement ants as in your photo usually happen during fights among competing colonies. Perhaps intruders from other colonies are only detected occasionally and that is when you see the ants above ground.

Pavement ants are also attracted by food and large numbers can also sometimes be seen when the ants are out foraging. So you may see more or less ants depending on whether they have detected a nearby food source or not.

Here are a couple links with more information about pavement ants if you are interested...
https://en.wikipedia.org/wiki/Tetramorium_immigrans
https://bybio.wordpress.com/2014/07/22/ant-wars/
https://extension.usu.edu/planthealth/research/pavement-ants

Hope that helps a bit...