I think Lynn is correct. I see a few semps from time to time where the growing part of the rosettes appears to actually be isolated from the root. The rosette gradually decreases in physical diameter, while the stem lengthens between the root and rosette. The plant appears to be feeding upon itself, as it grows taller but looses diameter. The older leaves dry up, being used for food to keep the rosette growing but unless you intervene, the rosette will eventually die.
The cure for this is to dig up the plant, remove all the dry leaves, then replant so that the rosette has contact with the ground. I seem to recall a very old thread here (or maybe it was Cubits) showing some pics of this. The rosette will recover, but take some time to get large again.
Your noticing that the soil eroded away from the root (or some other cause) suggests to me that the plant just was not getting either enough water or enough nutrients to the rosette from the root.
