It could be one of three things... First, the material that Hunter used to fair the hull-keel joint has fallen out/shrunk. Second, the keel bolts need to be tightened or replaced. Third, the keel support structure in the boat has weakened and needs to be reinforced/repaired.
Is there any rust discoloration of the crack? If not, then it may just be that the fairing compound used to fair the hull-keel join is either missing or has shrunk.
However, if there is any noticeable corrosion or rusty staining, then you probably need to drop the keel and have a look at the keel bolts. These bolts are almost 20 years old and may be in need of replacement at this point in time. Rust colored stains at the hull-keel join would indicate that the ballast or bolts are rusting badly. Is your keel iron or lead??
This is a photo by Maine Sail of what can happen to keelbolts, and why I say they need to be inspected.
Might also be worth looking in the bilge to see if there is any noticeable change in the hull and stringers that support the keel and keel bolts. On some of the older Catalinas, the keel bolts passed through a piece of glassed-in plywood, and on many boats the plywood got wet and rotted out...leading to the classic "Catalina Smile". Something similar could be happening on your boat.
My suggestion would be to have a surveyor come out and do an inspection of the area to start with...and work from there.