It was already established that the Runecrafting altars are all in pocket dimensions (except the Ourania and Astral), and the ruins we encounter are the remains of the destroyed temples for the Runecrafting Crusades.
It would be safe to assume the Soul Altar temple was destroyed at some point.
As for the Altar itself, it is quite possible that someone could have used powerful magics to move the Altar. Either consciously or subconsciously over the years.
It's also possible that the Soul Altar, without the Temple to anchor it, the altar decided to start going to a place that was a bit more natural for it. Soul energy .... underworld ...
It could have been gradual over years, moving at a rate like that of a glacier, only arriving in the last year or so.
It could have been quick, like when you cut a tight rope or rubberband. Once the Temple was destroyed, the Altar could have gone almost instantly to the Underworld.
22-Mar-2016 16:44:23