Why is my paint peeling?

A marvelous question. Why, oh why, IS my paint peeling? At least - some of it. Mostly in the hallway.

On Sun I spent three hours scraping off loose paint, creating an extremely ugly patchwork effect on the upper part of the wall. I was a little unsure what the next step should be - do I have to patch all the spots I scraped, as that would almost be skim coating? Can I just feather the edges? How come it's peeling????

So I applied Google. And discovered that in the absence of a leak, the problem sometimes occurs when the house has several old layers of oil paint and is then covered with a latex paint. Sometimes it's because there was originally calcimine paint used ("Since calcimine is essentially a water-based mixture of chalk and glue binder, in time any modern paint applied over a calcimine base will fail." courtesy of http://www.oldhousejournal.com/cures_for_calcimine_ceilings/magazine/1015).

The question now is - what do I do with the areas where the paint is still firmly adhered? I'm thinking I'll just clean what came down, seal it with a nasty sealer, and then maybe just feather the edges. But that might make a really wavy wall... Anyone dealt with this one?