Booking.com has a setting called "Auto Replenish". This setting causes a room to open back up when a booking.com booking has been cancelled, and it's totally independent of the channel manager and/or PMS.
When a booking.com booking is cancelled on the booking.com extranet by a guest, booking.com will automatically open the room and rate for the dates that the booking had been previously booked under, even if those rooms/rates are closed by the channel manager. This can lead to unexpected availability and thus could potentially lead to an overbooking.
Changing the Setting
This option is normally enabled by default but is agreed on in the commercial agreement between the property and account manager (Booking.com). If a property wants to change this, the property must ask their booking.com account manager.