Overwatering, too cold, or too hot.

My money is on "too hot".

Even if you get the air temp down around 80 or so, the soil in your flats could be significantly hotter. I have never grown in a green house at all. However I have routinely started seeds in sunny south facing windows. And I have discovered that not all sunny south facing windows are equal.

Where I am now, the sun in my south facing windows is great. So great, in fact, that even though it is October and somewhat chilly out (relatively speaking), the sunward side of my seed starting flats heats up to well over 100F - I measured it at 106F and then panicked.

I have since shaded the sunward side of the flat with folded white paper towels and this has reduced the soil temp on that side by 10F to 15F (over what it is with no shading).

But the air temp isn't much over 78F to 80F. It is the effects of direct sunlight on the flat itself.

My money's on the 100+ temps you were reporting.