It's a fungal infection of seedlings
From the University of Wisconsin:
"What is damping-off? Damping-off is a common and fatal disease that affects all types of plant seedlings. The disease is most prevalent when seeds are germinated in cool, wet soils. Fortunately, seedlings are susceptible to damping-off for only a short period following emergence and eventually outgrow their susceptibility to the disease as they age."
More information here
https://hort.extension.wisc.ed...
