I can see two possibilities (not mutually exclusive) :
- a sort of Male Rising scenario: Islam is seen as the "authentic" faith of the Black slave population and offers the framework for their liberation.
Come *ACW, big Islam-inspired risings supported by Union forces occur in most slave states. Subsequently, Islam is seen as an integral part of the new American identity and specifically a feature of Afro-American identity, spreading fast across a significant section of the population, mostly but not solely black.
- something horrible happens to the Ottoman Empire in a context where the US are encouraged to take Muslim immigrants from there.
25% is still very tall order, but these are the most likely ways I guess.
The first one sounds rather interesting, I would have to wonder how exactly one would bring it about. I am also curious as to whether this would have an effect on settlement in the west. Afro-American Muslims heading out west could end up doing a bit of proselytizing and bring more converts in.
I'm trying to think what short of a revival of the Byzantine Empire would be horrible enough to send muslims from the Ottoman Empire to the US. Hmmm...although I have to wonder if there would be some way for the Ottomans and US to end up having earlier relations, and whether this could somehow effect things.