Help my leaves fell off

Hi, I'm a pretty new grower. Trying to get a second opinion on this issue as well. So I woke up this morning to two of my plants looking like this. Rest we're fine. All on the same watering schedule/feed. I suspect underwatering but I want to be sure. Just happened so fast. Any help or second opinions would be much appreciated. Thank you all!

Maybe tell us about them a little. What medium are they in, what is the feed and watering schedule? What type of stress have they been under to have so few leaves and so many fall colors on the leaves they have?