Hard to say. Maybe underfed and/or overwatered. Did you follow the planting instructions exactly? Those are nice planters. You must have leaves to spare if you removed those. How big are the plants?

Followed the instructions. The plants are about 4 foot tall currently. I see more of these towards the bottom.

Also I have planted 6 tomato plants (6 Roma variety) in one of those planters. How often should I water them ? Also I used Mater Magic as tomato food.

6 plants sounds like alot for a planter that size, 20" x 24" x 9" . That might be your problem. They are stressing from overcrowding and the bottom leaves are dying. I might have planted 2 plants max, not 6. The containers are self watering, so it's kinda hard to overwater, just keep the reservior with water in it.