What reason would a hidden, deep ocean society have to come up to the surface of a post-apocalyptic Earth and claim the land for themselves?
I can think of only two offhand... revenge or reclamation.
Revenge... they don't intend to claim the surface for themselves, after all, it is: horrible, hot, dry, the pressure is all wrong, the sounds are all wrong, the light is blinding and the sun is burning, the food is weird... the list goes on forever. The surface is such an utterly alien environment that they're just not interested in it, just interested in getting rid of the stupid primates who live up there to make sure that they don't get the opportunity to do the same stupid awful things all over again.
They don't want to be there, they're not going to stay there for long, and the sooner they can get back down to the deeps the better. They're angry at you personally, for ruining everything and making them come up to this arid hellscape, and they're going to make you pay.
If you've not already done so, there's relevant reading in The Kraken Wakes or The Swarm (the one by Frank Schätzing) or Peter Watts' Rifters series (available as free ebooks from the author's site).
Reclamation... they used to live on land, or perhaps on the coast, but were driven away by human expansion, suffered badly from disease and pollution, gained a sort of mythical status rather than being taken seriously by the bulk of humanity and were ultimately scoured from the surface. Or maybe they were human, modified by some technological process or experiencing some kind of (at least partially) benign mutation, and were able to retreat to the oceans when the surface humans rejected them. Previous attempts to return failed, but not this one. This time the humans have been weakened beyond the ability to effectively fight back.
They may have adapted to their new undersea homes, but they came from the surface before and they've always wanted to return. Re-adapting to the surface world will be difficult, but they're biologically equipped for it and it'll come back to them in due course.
Perhaps they're escaping from an ancient Lovecraftian entity [...] Or maybe their food sources are running out due to widespread extinctions in the overall ocean food chain, and they're forced to come up to the surface and try their luck there,
Probably not so much... The ocean is huge, and the surface is utterly alien and extremely hostile to native undersea species. They'd move into shallower seas or coastal waters, at most. But to move from the abyssal plains to the surface? About as likely as you wanting to make the opposite journey and live in the dark and eat weird shellfish for the rest of your life.