Most of my tomato plants are doing really well. But the one in the cage highlighted in red started failing a few weeks ago. I fertilized with some Miracle grow I had lying around and it didn't improve. Then I got some Tomato Tone. That did nothing. The plant kept getting worse and worse and now it's just a husk. I'm afraid blue is following next and I'm concerned about purple. Here's another angle for some more context. 
.
Any thoughts?
I doubt it's over watering, due to the fact the others look healthy enough. Dig up the root ball of that one, look for pests in there would be my first step to diagnosing.