Ouch! Road salt or sea air to blame, I suspect. Good find. Fortunately I learned early in my Harley owning days the benefits of braided hoses, which I suspect feature in your imminent future!
Mine did that at about 10 years of age. You could see the swelling under the shrink tubing. Mine was never exposed to road salt or sea air, but rainwater gets underneath the shrink tubing and causes rust, as the metal hose ends stay wet.
I check the replacement lines regularly, but I've not seen any recurrence yet.
Yep...new lines installed, the amount of crap inside the master cylinder and caliper was incredible. All clean now and working as it should but a right P.I.T.A