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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?
Check the roots, you might have pests eating the roots. Could also be too much watering. Hard to say from photos but I definitely think something is wrong below the surface
I water almost every day. How do I know if it's too much?
When it shows signs of over watering, they only need about an inch a week after being established. When they are new or transplanted water daily for a week, than give it an inch of water a week. If it rains an inch, you’re done for week unless it’s stupid dry and starts showing signs of needing water (obviously).
You could always get a moisture probe. I've found it extremely useful
Every day is probably too much, unless you're in a dry area and they're very well drained. I'd describe watering as saturating, not flooding, and let it dry to the roots, not just the surface, before watering again. Dry, but not completely moistureless bone dry
Also, on hot sunny days, water in the evening or early morning, else you'll just lose a lot of water to sun evaporation.
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.
At first I thought it was one of the "wilts". These are soil-borne pathogens that attack the plants roots. The causitive organism could be verticillium, fusarium, or phytophera. In small plants pythium or rhyzoctonia can kill them. There is also bacterial wilt that causes the rapid decline of the plants.
Then I zoomed in and took a closer look at the plant. I suspect it's nitrogen deficiency. It could be caused by over-watering (denitrification and leaching nitrate out of the soil profile). However I suspect you didn't have enough to start with.
The tomato tone looks to be a 3-4-6 fertilizer. To put it simply, it's a stupid fertilizer blend. Plants need nutrients with a ratio of around 3:1:2. So you need 3x+ more nitrogen in that blend.
Once the plant sets fruit, it starts to dedicate nitrogen into the fruit/seeds. In a shortage situation it moves them from the lower leaves (they turn yellow and die).
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
Nah usually it's my fault.
Since you tried nutrition I would suspect injury to the plant has allowed some kind of infection in. I would try trimming away all of the diseased or damaged plant matter and then applying a copper fungicide. I've had tomatoes with vines split fully in two and the plant stayed productive and vibrant once it got far enough from the damage and new growth was happening.
"Late blight" possibly.
This is not a criticism , but they seem crowded, we all work with the space we have. Also, have you planted tomatoes in that spot before? That can cause issues too.
Don't over water, let them dry out a bit, remove the dying plants asap, use a copper fungicide, and next year replace a lot of the soil if you have to plant in the same spot. If you can, rotate your crops every year.