The weed seeds were already there before you put in the sod. When you tilled, you brought the seeds up to the surface which germinated when the conditions were right (I suspect you didn't put down pre-emergent in the spring?). The weed killer wouldn't kill seeds. Some of the seeds could also have blown over from the neighbors'.

Rototilling was a mistake for that reason but also because as the fluffed up soil settles, it will settle unevenly over the next 3 years giving you a bumpier and bumpier lawn as time goes on.

Nutgrass loves swampy conditions. If you are watering more frequently than once every week or two, then you care helping the nutgrass. Similarly crabgrass loves getting daily water, especially when the seed is germinating in the spring. In the early spring, once a month water should be sufficient until the temps get into the 70s all the time.

Toward the end of this month you can spray the nutgrass with something like Sedgehammer or Image for nutgrass. There's a spray for crabgrass, too, but the name escapes me. Kill those out and bring in either new sod or seed for the fescue. Don't wait for spring to do that. Fall is the best time of year for new grass in the north.

My new celebration Bermuda sod came with both crab grass & nutsedge from the sod farm. I knew it would so I caught it early, but I now have nutsedge in areas I never had it before

Drive XLR will knock out the crabgrass, or pretty much any herbicide with quinclorac.

I should have added that after tilling I hit the area with a whacker jumping jack compacter. I rarely water but we an early spring & have had unusually wet weather. there was a heavy rain last night. I got out early & pulled a of both weeds. the other battle i'm fighting is at the back of my property. creeping Charlie is taking over there. I found a source on ebay selling 3.3 oz of sedgehammer ( enough to make 40 gallons ) for $87.00 which beats all local suppliers.

I don't think you can use Drive XLR8 on St. Augustine.

I left the St Augustine behind when I left nawlins'. I have fescue & I'm thinking about over-seeding some K.B.G. the best advice I've seen for nut sedge is to move. the thing I hate about the chemicals that kill the crab grass & creeping Charlie is that they kill off my clover.

I think I got my threads mixed up, lol... But I've had good success with xlr8 on crabgrass in a fescue/KBG blend.