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. 
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Any thoughts?
No APPARENT reason. :) They won't die for no reason. I'm with the other folks, some outside agent, a pest or something.
It might not even be bugs, you might have a neighborhood critter picking that spot to urinate. I'd set up an outdoor camera.
No, I was intentional about word choice. There must be some reason, I just have no idea what it is