The two containers of the same lettuce, watered the same, same growing medium, and same sun exposure, are very different as you can see! What do you think killed the lettuce in the near container, and what should be done about it? Thanks.
Was the planter shown in the front covered with anything (e.g. plastic?) for a period of time? If so, too much heat may have "cooked" the lettuce. If that wasn't an issue, did the affected container have poor drainage? Water saturation will kill roots and cause complete dieback like this. Those are a couple possibilities.
Despite the problem with the lettuce in that particular container, we commend you on your garden setup! It looks quite nice with the galvanized containers and the lettuce in the far container looks very healthy. Our vegetable extension specialist would be curious to see additional photos of how you set up your container garden -- if you are willing to share.
If the growing mix was saturated with water for a period of time it's not unlikely that a) plant roots were starved for oxygen and b) one of the "damping off" pathogens like pythium or phytopthora, infected the roots and possibly the stems causing rapid dieback of the lettuce plants.
Awesome container garden set-up! jt