Just guesses, but two things that turn phlox brown are, as you suspected, too much water and also too much fertilizer (or both). Probably the water...but ground covers should be allowed to establish for a month or two before a even a light feeding, just in case you gave them a shot upon planting. Phlox likes a slightly acid soil, so if your soil is of a high pH it can affect them, too.
One more thing, if they got too dry before planting they can scorch and brown out after planting.
Good news - phlox is pretty resilient, so hang on!
Hortster

Thanks Hortster. I hope my phlox will bounce back. I'm a newbie, so any browning or wilted leaf worries me. I will have to learn the balance of over and under watering! Thanks again!