I can't comment specifically on SLC (or even the TSA), but every major airport worldwide (and some minor ones too) that I have dealt with has someone on staff whose sole raison d'etre is to develop a passenger presentation profile and model to determine staffing levels at terminal choke points such as security.

That said, passengers do not always adhere to the presentation model. A handful of tour groups arriving at the same time can flood a choke point very quickly. A forecast of bad weather that doesn't hold up is another common cause as people tend to show up early and then hang around. Similarly, just a handful of sick calls can result in significantly reduced screening resource being available.

There is a science behind it, but like any prediction, the only thing certain about it is that it will certainly be wrong.