It's the nutes, and that is clearly overfeeding. Lower leaves shouldn't be dying off already. That commonly happens to an over-vegged plant and a plant that hits flowering. When they are that young, they shouldn't be losing any leaves yet.

Ease up on the nutes, and only feed 1/2 strength once a week, but for the time being, water with clean water until the burning/yellowing has stopped, then start feeding again.