Elder Dallin H. Oaks explained this phenomenon well in a talk last year. Here is an excerpt:
"Persons who don’t believe in God or in absolute truth in moral matters can see themselves as the most tolerant of persons. For them, almost anything goes. This belief system can tolerate almost any behavior and almost any person. Unfortunately, some who believe in moral relativism seem to have difficulty tolerating those who insist that there is a God who should be respected and that there are certain moral absolutes that should be observed."
Elder Dallin H. Oaks, "Balancing Truth and Tolerance," Ensign, February 2013, https://www.lds.org/ensign/2013/02/balancing-truth-and-tolerance?lang=eng.