The area that's hollow is where the "pith" would normally be. The pith is the soft, spongy center in the middle of plant stalks where carbon is stored. When the pith starts to disappear, it's called "Pith Autolysis" and it does NOT indicate any real problems with the plant.
What it does indicate is that the plant needed more carbon, so it started to feed on its own carbon reserves.
Some plants are genetically prone to have a hollow pith and other reasons can include plants that are grown in low light conditions, low CO2 conditions, and fast-growing/leggy plants can also be more prone (as
@Duggan pointed out!).
So, for example, if the tent is packed with plants, there may not be enough CO2 for all of them because they're all sucking it all out of the air. So, because there's not enough CO2, there's not enough carbon, and because there's not enough carbon, the plant will start feeding on it's own carbon reserves in the pith.
Low light conditions effect the plants ability to photosynthesize, so even if there's enough CO2, they won't be able to break it down, resulting in the same feeding on it's own carbon reserves.
It does not indicate any problems, per se, just a sign that the plant was feeding on its own carbon reserves.
Basically making the plant follow the outer edge of the pot
