What causes skinny roots?
Thanks for the question.
This looks to me like a condition called “damping off”. If is caused by various types of fungi and typically occurs at the soil line. This leads to a destruction of stem material that is causing the thinning of the stem. In turn, this impedes the transport of nutrients up the stem which is probably responsible for the leaf yellowing you are observing.
These seedings will not recover and you will probably have to start over. This time consider using planting trays that water from the bottom up. Damping off is most symptomatic of over watering. Also consider sterilizing your planting trays.
Here are some sites that will give you information on these and other points: