I'd want to know more about the consistency and ingredients of your potting mix, as well as your fertilization practices. Only you can determine if you're watering or not. Are you testing the medium with your fingers before watering or going by the appearance of the soil surface? Do you water a certain number of times a week no matter what?

Enjoy your kale!

Are you fertilizing it ?

I don't think over watering is a problem with brassica as long as they have nutrients.

ALSO, yellowing lower leaves is normal. That is the plant's abortion system, to maximize itst growth.
One more thing, In container growing, yellowing lower leaves is more common than in ground. So try to pick them early.

Do you fertilize? Needs nitrogen.

Thank you everyone for the replies. I removed the yellowing leaves and other haven't taken their place, so that's good! I haven't added more fertilizer since I planted them, so it sounds like that's the next thing I should do.