A bit more has come to light from various articles. The impression seems to be that Lucasfilm was (in addition to their other issues with Lord and Miller) upset at how the Han Solo character was being developed. The implication seems to be that whereas Kennedy and Kasdan wanted something akin to a Western or heist movie, what they were getting was way more overtly comedic than what they expected, and possibly, they felt, not particularly respectful of Han Solo as a character. But it's so weird it took them this long to pull the trigger. I guess they just kept trying to make it work.
Rumored list of possible replacement directors now includes Ron Howard (alleged frontrunner), Kasdan himself, and Joe Johnston.
seems to me that it's b/c once 9 is done, they won't go back to the skywalker story - everything will be the standalones, so that's what they really want to control
Apparently they're still trying to figure out the way forward with that. We may know some more at the D23 presentations later in the summer. I'm fine with either, personally.