Let us remember what happens when Tom first reaches home after his encounter with the devil. Having just sat next to a rotten stump bearing the name of Absalom Crowninshield, he comes home to be greeted by the news that the same person, infamous for his bucaneering, has just died suddenly. This serves to make Tom feel that all that had occurred to him was not a daydream but was actually real and true. The story itself thus explains why Tom feels inclined to take his wife into his confidence:
He was not prone to let his wife into his confidence; but as this was an uneasy secret, he willingly shared it with her.
Thus we can see that the nature of this "uneasy secret" meant that Tom overcame his normal suspicion of his wife and felt he had to confide in someone and tell them what he had experienced.