The leaves may have gotten sunburned...did you place the plants outside after growing them indoors (?) especially after watering from the top,because the water drops can act like magnifying lens to accelerate sunburn...it's also possible that the plant is naturally withdrawing nutrients from the older leaves to re-locate to the newest growth...

Ironically, the plants get very little sun. They stay indoors in a northwest facing window and only get full sun at the end of the day. Somehow they seem to grow quite rapidly even in this mostly shaded area.

I didn't know that plants could draw nutrients to promote newer growth, that is really cool! Thanks for the info. I'll keep an eye on the plants and post if anything dramatic happens.

Might be overwatering