Plant leaves turn yellow, become chlorotic, because they loose the chlorofil that makes them green. Why that happens has a multitude of reasons ranging from the soils pH to the soil moisture levels and just about everything in between.
Perhaps the link below can be of some help.

Here is a link that might be useful: About plant chlorosis

Thanks, that may help. I noticed today some of the first leaves to yellow are now wilting which makes me think it's a root problem. I'm just hoping I don't lose the plants since they were doing so well to begin with.

This sounds very similar to what I was experiencing with my butternut squash plants. I think I was overwatering. I still water every day or every other day due to the hot and dry climate I live in, but have backed off on watering so deeply. I also fertilized the plants twice this month, two weeks apart, and they are now in full recovery. It took the 2nd feeding to see a big difference, but it has been quite dramatic at this point. Maybe feeding the plants a bit more frequently will help yours, too.