Why did Tituba and Christopher have a falling out?

Christopher does not truly love Tituba. He only wants to use her magic powers to make him invincible. He sleeps with her, treating her like he has treated so many women before. To Christopher, Tituba is little more than a sex object:

A woman's duty, Tituba, is not fight or make war, but to make love (152).

Tituba is a caring, compassionate soul, and she is vulnerable to being taken advantage of. Christopher's effectively exploiting Tituba's sexuality in much the same way as the white Puritans exploited her race.
Christopher also has delusions of grandeur. He genuinely feels himself becoming immortal. He starts imagining he can hear slaves singing songs about him as they toil in the fields.
In truth, however, Christopher is a complete coward. He has already made a grubby, underhanded deal with the plantation owners to alert them to any potential uprisings. As Christopher no longer has any further need for Tituba or her magic, he betrays her and Iphigene to the authorities. Both Tituba and Iphigene are subsequently hanged, but their spirits return to incite future rebellions. Tituba's spirit haunts Christopher to such an extent that he loses his appetite for women.