Multiculturalism is not simply a juxtaposition of differing cultures.
It is a culture in it's own right, with it's own central principles to which participant cultures agree to submit.
Participant cultures are rarely by origin multicultural. Often it requires certain adaptations on part of the cultural adherents for a culture to become compatible with a multicultural society.
There is a vaguely defined and arbitrary distinction between public and private, a line that may be drawn differently in different multicultural societies, which requires cultures to cede the public realm to a common neutral ground.
Incompatible cultures which are reluctant to adapt, especially when the incompatibility is seen as a reflection of authenticity, and/or are reluctant to cede the public sphere, would come to see multiculturalism as a threat and they would in turn become a threat for the multicultural culture.