Why are some mountains forming at non-plate boundaries?

There are several mountain building processes that are not associated with plate boundaries.

Block mountains for example does not require any of the condition at a plate margin to form. So far the crust begins to elongate by extensional forces, rock blocks drop and some other rise to form series of horsts and grabens.

The most prominent mountains that forms outside non-plate margins involves sedimentary rock formations.

Here, sediments accumulates within a basin where they get compacted and lithified. During episodes of regional uplift, the crust might rise to new levels and the basins would become elevated to towering peaks.