i think its a light deficiency. it's no worries at all. you're going to want to trim a lot of that stuff at the bottom anyway. it's hard to explain for me. the small stuff at the bottom sort of robs the plant of peak productivity since its shaded by your canopy. its still alive, therefore using valuable water and nute resources. So anything and the bottom of your plant is okay to fall off. I think the plant might just be doing what it does best and letting go of what it doesnt need anymore.