I recently set my calendar for 2 week availability by blocking all the days in the month for which I could not offer availability. One of my guests made a reservation for 1 of the weeks available and somehow that caused the calendar to open all the remaining days that I had blocked causing another guest to pick some of the days that were previously blocked. I beleive that it may be the result of a software glitch that should be addressed because it has the potential of causing a lot of confusion and conflict.
Welcome to the joy's of airbnb's magic calander that opens dates at random, only happened a couple of times to me but we have lots of posts on the forum about it.
Watch out for the bugs, we have lots of them.
I notice you cancelled a booking, If its the bug that caused you to cancel, call CS and get them to remove the penalty.