How do these things seem to get turned around?

Time basically.

Some of it is because of Irony. Similar to how people in the 80/90s use the word "Bad" to mean "Really Good", other words changed meaning because of ironic usage.

Egregious used to mean "rising above the flock, exceptional, distinguished." People kept using it ironically so much that it now means exceptionally bad or distinguished by being particularly bad.

Other words have had a more gradual evolution to their opposites, like "Nice". It originally meant foolish or weak. During the middle ages it came to mean shy, reserved, or fastidious, but those qualities were still considered 'weak'. In the late 1700s society began to see merit in those qualities and so being 'nice' was no longer a foolish or bad thing to be.

Same thing with phrases and idioms. Hundreds of years can have a weird effect on language.