Just recently starting growing microgreens. I had 2 trays that turned out excellent. My last tray and now this current one are doing as shown.
Look at the bottom of the stems near the soil
That looks like a seedling fungal infection to me. It could be numerous different species.
Seeing some fuz, but nothing that jumps out to me as more than root hairs. But maybe I don't know what I'm looking at. Trimmed it out to try and get a better look.
Pull out some plants. What you are looking for is an unhealthy looking portion of the stem near where the soil meets the stem. It can be above or below the surface.
You might need a magnifying glass or use the magnification lens of you phone camera.
This is the full rack. The bad side was up to the wall, but I don't know if that matters. 
Maybe more light towards the front? Other trays look weaker the further from the wall they are as well.
I probably do need another light, seems most people use 3 and I only have 2. I don't think that would cause the issue though, probably just less growth.
Sprouts are very sensitive to light, temperature, and humidity.
Is that wall cold? Are there shadows where it sits? Is it warm in that area, or just filled with light? Do you have fans running? Is there an automated watering system providing the EXACT same amount of water to the "good" side vs the "bad" side?
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