Why is Reverend Parris so terrified by the events in Salem? What possible result does he fear?

In Act I, Abigail warns Parris that there are rumors of witchcraft and that the parlor is packed with people. Parris tells her that he cannot reveal that he found his daughter and niece dancing in the forest like heathens. Abigail admits to dancing and is willing to accept the punishment, but will not admit to witchcraft. Parris warns Abigail that he has enemies who will use this situation against him.

In actuality, Parris wasn't the least bit terrified by events in Salem since he instigated them. But yes - Abigail Williams did dance under the moon. Did he know it? No.

It's simply a matter of correcting history.