From our tomato expert:
Mostly heat and moisture stress. Keep root system moist consistently. Move plants to afternoon shade on scorching days.

Take a look at info on the link:

Tomatoes | University of Maryland Extension (umd.edu)

I'd remove lower browning leaves, being sure to sanitize pruners with bleach between cuts.  If lower leaf browning continues, consider spraying with a fungicide. Read directions.

Next year, be sure to sanitize containers with bleach before filling with new potting soil, and plant tomatoes varieties with disease resistance (to blight, wilt, etc...)

Good luck
Len

Thank you!