@Jessica2159 The possibilities are that the guest's payment fell through, there was a report of them trashing another host's home, carrying on illegal activities at a prior Airbnb, or their ID verification was found to be fake, etc.
Although there are occasional glitches, if Airbnb unilaterally cancels a booking, there is probably a good reason for it. It's one of the few ways that they protect hosts, and it's a good thing.