That is a strange little bugger. I have had seeds sprout upside down before, not often, but around 1 in 100 or so will do it (depending on the genetics, of course). When they do sprout upside down, normally the root will redirect itself into the rockwool by curling back and inserting itself back into the cube for anchorage, leaving a corkscrew in the lower stem. If I notice a root emerging first instead of a seed or cotyledons (first pair of round leaves), I gingerly remove the seedling and reposition it upright. Be very careful when handling seedlings, they are so much more sensitive than any other phase of growth. They won't always survive, and you need to catch them early; it looks to be a bit late to save your baby, but you may try peeling away the rockwool to see if there are still cotyledons alive inside. If they are green, try to save the plant. You'll know if they are dead, they'll be mushy.
Also, usually roots have root hairs coming off of them and I didn't see any signs of root hairs in your picture (good job including a good pic). Did the part that emerged first have fuzzy hairs on it at any point that may have died off?
High humidity can encourage roots to grow out of the medium, even upwards, if the medium is too wet. Make sure that the cubes aren't saturated, and acclimate the seedlings to normal humidity levels, over at least several hours. Ask if you don't know how to acclimate. Excess humidity during the seedling stage can also cause seedlings to stretch more than is healthy for them. Often, they stretch so much that they can't support themselves and collapse. It may be wise to support your seedlings somehow (I use a toothpick stuck into the rockwool with a twist tie around it, then bend the twist tie to support the plant without damaging it). I wish I had a picture, but you'll figure out a way to support them, us growers are innovative and creative
Please do make sure to take good care of those girls, a greenhouse mix pack gave me 2 exquisite mother plants, and I have head amazing things about AMS, AH #3, and the Rhino.
PS: You can still mist your seedlings several times each day, but make sure the leaves dry between applications or you may have problems with mold.