Happened to me as well in December on the 2nd night of a 5 night stay at a Fairfield Inn. I received an email on the second day of my stay showing the reservation had changed and an additional 2,500 points deducted - my guess is that the property upgraded my room which triggered it. Spoke to the front desk the next morning, and they said they had nothing to do with it. Spoke to Marriott and they said it was because reward nights were booked in September prior to the peak night award changes, and that the additional points deducted were the extra cost of that night based on peak rewards. The agent agreed to reverse the extra points which was done almost immediately. I'm wondering if this is caused by reservations booked prior to the September peak/off-peak rewards changes, and then the property changing the room from the original reservation triggers the reservation to be changed to the new rewards chart.