I used to actually know the various chemical and light triggers that cause this but I long since forgot.

Edit:

So I seemed to recall that ethylene gas was a major factor.

Yup. Which then led me to this.

This is a very good starting point.

https://untamedscience.com/biology/plants/plant-growth-hormones/

Roots don't have to go very far, roots are going in search of water and nutrients. If they do not need to grow to achieve the nutrient mass required to feed the leaves then they will essentially slow down. Normally the root mass equals the upper half of the plant mass but that is not in the case of hydroponics because they don't have to go to work to get their food or water.

My problem with hydroponics is you have to pay so close attention to the pH balance and the parts per million of the nutrients and each plant has a different optimal level. What's perfect for one might be totally wrong for another. If your water gets too hot or too cold overnight they will all die. If you use too much of any particular nutrient they will sick and some will die die. Dirt is much more forgiving. Dirt is essentially a living buffer for anything I might screw up.

It's been a couple of days and the turnips are all popping up as well as some broccoli and some cabbage and some lettuce.

I initially thought I horribly screwed up because I mixed the mycorrhizal fungus in with the dirt in general because I did not have that much of it. Typically I drop a couple pellets in the seed hole and it all grows together with the seed and the roots. At this point I have white fungus all over the place that I initially thought I just have to trash it all over and then I realized it was my beneficial fungus.

Hydroponic setups typically have the plants firmly anchored between the stem and the roots. So the plant is held in position. Usually the roots go through a large basket holding the base of the root mass and the rest trail in the liquid solution.