There are a couple of reasons why room can become double-booked.
Direct Additions
A room can get double-booked if a person identified as a room owner has given others the rights to create appointments directly on the room mailbox's calendar (e.g., Open A Shared Calendar). This is because no meeting invitation is generated and therefore can't be processed.
Modifying a Reoccurring Meeting
A room can also get double-booked if the room has accepted a meeting for a specific time but then a different reoccurring meeting is changed to include the room after the space is already booked. Because the room can't decline the entire series, it will double-book the space.