Originally posted by Renlish:

You've spawned there because your bed no longer exists. Either you destroyed it (or it was destroyed in a raid) or you might have rebuilt it but forgot to sleep in it to make sure you set your spawn point. This is generally the only way you can end up at the elder stones starting area.

Head back to your base or whatever area you've set up to return to and make sure a bed is there and that you've set the spawn point on the bed.