No special knowledge or insight on this, but -- if you look at the projections on most European maps of the Crimean area, Sevastapol seems to be just an enclave off to the side. About a year ago, I noticed an old Russian naval map that put the port at the center, and it makes a very big difference. The mineral resources and ethnic populations are of course factors as well, but if you look at the land taken in the present conflict, there seems to be a good chance that the incentive to do so came from trying to protect the place where they keep all the boats.
Another 19th c. view: