I have heard that the hotels review the number of times that someone uses a certificate to stay on points. Thus if a hotel sees an upsurge in certificate use (for which they get very little reimbursement) then a category change (upward) is begun.
I have no proof that this a actually the case, and leave it to Marriott to explain the changes.
Thank you for clearing this up. I was hoping it had to do with the quality of the hotel since the pricing tends to rise with the category type. I didn't understand how the same hotel improved 2 categories over 2 years