If Sikh immigrants flock to an area and setup roots there, it's a Sikh stronghold. We'd call it a Muslim stronghold if the majority of arrivals were Muslim.
Is this where you get a bit touchy because you're not the physical type that fights back?
Who said it is a "stronghold?"
Just because an area has relatively more Sikhs than other areas, that doesn't mean that Sikhs have large numbers there. Sikhs have tiny numbers everywhere in the US. They are just slightly less tiny in certain areas.