Seedlings are highly susceptible to dehydration, and losing water faster than they can absorb it can be a quick way to kill them. This can happen due to several factors, including insufficient watering, excessive heat or dry air.
Capacity to store water is held in the stem, so until she has capacity to hold more for longer, she runs a fine line between what she spits out and what she uptake, there is little to no room for error. Even a little too much or the hot light beating on the soil can dry those top couple inches very fast, gravity does its best to pull the moisture out of reach creating a potential for quick death.
I don't know what your soil composition is, but transpiration is the process responsible for building turgor pressure, it is this counter pressure which is used by the rootzones to penetrate soil search for minerals and water. If it's too low its possible she couldn't penetrate hard compressed soil, but given your experience I doubt that to be a cause.
Maybe she just got a little infection?. All I got .