The mountains become lighter, and float higher.
In our world, the Appalachians formed from plate tectonics in the remote Ordovician. But much later, in the Cretaceous, they rose again. Plate tectonics were not responsible for this late rise. Instead, the mountains rose because they became lighter.
Appalachians Get a Face-Lift From Earth's Mantle
The Cullasaja River basin, part of the southern Appalachians in western North Carolina, holds scores of mountains and valleys, but parts of its terrain are more rugged than others... Usually...rugged
peaks point to younger terrain with recent or active tectonic
mountain-building processes. But geologists know the Appalachians have
been tectonically quiet for more than 200 million years. What could
have rejuvenated the southern Appalachians?
Plate tectonics and the usual mountain-building suspects probably
weren't responsible, since the Appalachians were tectonically quiet by
that time. But by turning to previous surveys of the region, Gallen
found another possible culprit.
Regional uplift driven by the Earth's mantle — the hot, flowing layer
below the outer crust — could be the culprit, according to a team of
researchers led by geologist Sean Gallen of North Carolina State
One way to explain those features is that the dense "root" of the
mountain range delaminated, or peeled off, from the rest of the crust
around 8 million years ago. "Sometimes the mountain root becomes more
dense than the mantle below, and it's not gravitationally stable,"
Gallen said. "It will basically drip off the base of the crust, and
the remaining crust, which is lighter, will bob up on top of the
The heavy bottom of the mantle falls away into the earth under gravity - "delaminates" and separates from the lighter upper parts of the mountains. These lighter parts then bob higher.
In the alternate world, the processes in motion in the Cretaceous continue to this day (and to some degree they are in our world). Perhaps more of the base delaminated and what was left above was even lighter. Perhaps the mantle below was composed of denser materials and the mountains floated higher.
In either case, this is a real world process that may be responsible for the real uplift of this mountain range in our world. Augmenting a real thing for a fictional world is a fine world building strategy.