It didn't take long for the conspiracy theorists to show up.
If you can't contribute more than just dragging up an author who was probably suffering PTSD after his Spanish Civil War experience then
off.
After having flown the A320 for quite a few years IMHO it is quite possible that the crew may have selected V/S instead of the altitude selector to commence a descent. In which case there is no altitude "floor" for the A/P to capture. Assuming they were incapacitated then it is inevitable that the aircraft just kept on descending.