brakes and boat trailers are a nightmare. We are lucky with the local slips in that with a bit of ingenuity we can keep the drums dry but once dunked its a one way downhill process . As said the rust gets twixt shoe and lining and prises it off.
Lucky you didn't seize a wheel up.
We had one fall apart in the storage yard and had a **** of a job to get the drum off. Wheel locked up solid with the loose lining jammed against the drum.
Auto reverse loves to seize up too once they have had the salt in there.
Welcome to the world of braked trailers!! -they are indeed the work of the devil!