Drupal hash salt / private key is changing?
drupal_get_hash_salt() is either:


a manually-entered value stored in your settings.php 
...or (by default) a hash of your serialized-DB config 


drupal_get_private_key() is either:


a stored variable called drupal_private_key if it's set
...or (by default) randomly generated bytes (that are saved to drupal_private_key after generation)


So, either one can change their value if:


You changed your DB config (if $drupal_hash_salt isn't manually set in settings.php).
You've lost/removed drupal_private_key from the variable DB table.