Any weed killer being sprayed in the area?

Nope, no weed killer at all. And the leaves haven't turned yellow or brown. They've stayed deep green for about a week to a week a half now, since I first noticed the change in the leaves.

Did you add anything to the soil?

Mine look like that, too. Healthy, except for the curl. A master gardener friend of mine said it may have something to do with lack of high temperatures. I don't know if that's true, but we have only had 2-3 days of weather over 90 degrees in KC, where I live, and none of those were in a row.

Damping off (too much water) maybe?

Looks like Leaf Roll. This can be a physiological indicator of root stress. Usually happens with overwatering plants but can occur for other reasons as well.

Try implementing a consistent watering schedule and perhaps reduce watering frequency.

Good mulching can also help retain soil moisture and improve the condition.

Could also be air pollution other than drifting herbicide.

My plants that are nearest our dryer exhaust vent get leaf curl when they're young. Currently they have outgrown it and are growing normally, but a couple weeks ago, the leaves came out curled and distorted like the ones in your pics. It happened last year as well, in the same spot.

So if you're near a dryer exhaust vent, or other possible air pollutant, that's probably your problem.

If it's not air pollution, then it might be leaf roll, as cirtes suggested.