I did some reading and while it's true that the continued existence of the US federal government is a large collection of dick moves forming one gigantic meta-dick move, this is actually pretty straightforward. The UN Convention on the Law of the Seas defines a range of distances from the seashore where a state can claim the seafloor/minerals etc as its own; everything past that is the high seas. The US hadn't previously maxed out its claims, so there was wiggle room under UNCLOS to expand said claims.

Now, why would they bother, why is it suddenly worth the extra administrative cost of claiming even deeper offshore waters, that's an interesting question. I'd say it's a good indicator of the increasing cost and difficulty of extracting natural resources (likely technology has brought the cost down some, too), pushing nation-states to pursue ever more exotic and costly extraction methods, but overall this doesn't seem that significant (we all already knew that was a trend, that's why we're all on this community).

The push to expand territories is a troubling one, because sure, this is a legally uncontroversial move, but if expanding territories is at this point our best option for propping up the system, we're in for another era of wars. But we all knew that already.