I put preprimed exterior casing around several of my windows and doors
several years ago. I topped it off with oil-based exterior paint.

At the same time, I put some new trim wood (not casing) on the
outside, used an oil-based primer, and an oil-based topcoat.

Now, 5 or 7 years later, the wood that wasn't preprimed is in fine
condition. But in some cases the casing that was preprimed is starting
to peel. And it's not the topcoat peeling off the primer.

Did I do something wrong 5 or 7 years ago by using oil-based paint
over the preprimed casing?

Or is it just that 5 or 7 years later it's time to re-prepare
(scraping and sanding as necessary) and re-paint anyway?

Tim.