Sounds like the piston was badly pitted from rust and it damaged the seal. I don't know what a mechanic could do to prevent this, aside from removing the outer boot and pushing the piston(s) most of the way out for inspection.
We're weirdos in our old vehicles. I'd bet that your average car (12.2 year lifespan as of 2022, up from ~10 years in 2010), even with neglected brake fluid, isn't old enough to have this kind of corrosion to the calipers and pistons. I doubt mechanics think twice before pushing the pistons in and slapping on new pads.
