There are two possible reasons I know of that could cause this to happened -
1) If you blocked via your calendar, your availability setting the minimum which I believe is for 'guest who arrive within 3 months' opens dates in your calendar to corresponding with this setting.
2) I have also read on here about a number of guests who have mysteriously had chunks of their blocked dates opened so looks like there is a bug as well.
I would suggest you contact Airbnb and explain the situation to them. They will be able to look at your account settings and will also be able to cancel for you if you are unable to accommodate the booking. Whatever you do, DO NOT cancel yourself otherwise you will be penalised; click on the link for contact options - Contact Airbnb
Also email the guest with a brief explanation and let them know you are currently liaising with Airbnb re the situation and will update them accordingly shortly. Airbnb will also contact the guest and attempt to find them similar alternative accommodation or offer them full refund if they are unable to assist – at least this way none of you are penalised.
I would also sugges you snooze you listing after this.
All teh best, Ama