Some possibilities:
More conversion to Islam among black Americans combined with smaller Great Migration leading to Muslim majority in Deep South.
A movement to convert to Islam begins in 19th century Upstate New York (I'm thinking analogous to OTL Mormons) and they settle and dominate a western state comparable to the OTL Mormons in Utah
More immigration from Muslim countries is the more obvious suggestion. But for a somewhat odd version of this, have the US annex Hawaii but not the Philippines, and then Hawaii gets a large wave of Indonesian immigrants.