sudden collapse of healthy broccoli plants

About two weeks ago I set out 80 of home-grown broccoli plants under row cover fabric on hoops. The plants had been grown in soil blocks and are being irrigated with soaker hose. I was worried about cutworm damage since other beds had heavy cutworm pressure, so I put a collar around each plant. There had been a lot of rough organic matter turned in to the soil about two weeks before planting, and I had fertilized with soy meal and gypsum (heavy clay soil.) I also gave each plant a dose of fish emulsion. The plants are now about a foot tall and growing rapidly. Over the past few days, about 15 of the plants have suddenly collapsed, wilting away, with the older leaves turning a reddish purple. The process seems to take about three days. The remaining plants still look the picture of health. I at first assumed that I wasn't watering enough and that the plants were not yet connected to the soil capillary; the ground was rather rough and the plants were in large soil blocks. However, heavy watering has not solved the problem. I dug up some of the affected plants, but haven't found any signs of critters or clubroot. The stems do change from green to brown and somewhat narrower right under the soil surface, but the healthy plants looks the same.

Anyone have any ideas?

Also, the ones that died are localized in several patches throughout the beds.

It could be Wire stem which is caused by Rhizoctonia spp
an article on it can be found Here
Early symptoms are a purpling of lower leaves, followed by wilting and a thin wiry brown stem at the bast where the stem meets the soil.

Looks like you nailed it. It is worst in wet, heavy soils, with recently tilled in un-decomposed organic matter and lots of nitrogen, and soil hilled up around plants. Check check check check check. (They were hilled up to keep them upright.) And the pictures/descriptions seem similar. Unfortunately dealing with Rhizoctonia can be difficult.

It is really hard to get compost piles to work here; they just dry out so fast. Last year, I started just spreading all the debris on the surface of the soil, chopping it up, and then digging it in before planting. Looks like I won't be doing that again. I might dig it in to the soil in the Fall, and then cover with black plastic to speed composting.

Now I'm trying to figure out; for plants that show the constricted stem but haven't collapsed yet, should I hill them up so they can grow new roots out above the damage? Or would that just make it worse?