I've never sprouted seeds using rockwool, but I know you're supposed to soak them in water with a certain pH before using them. Could that possibly be the culprit?

Also, no extra nutrients until the plants have at least four nodes. It's easier than easy to burn such young plants. Soil can also play a big role in burning seedlings if there are a lot of nutrients premixed, but it doesn't look like those seedlings have dug all that deep into the soil yet. Also, be careful about how much flushing you do when they're that young, as that can invite a mold commonly known as "damping off", which is often fatal to seedlings.