I think they realized around season five that they weren't going to be able to hold onto the whole cast for long enough to finish the show properly so they sped things up a lot. they also caught up with the books around here so they had to start making stuff up themselves.

this is a classic problem with long running tv shows. it's hard to get the pacing right when you don't know in advance how many seasons you're going to make.

That would be new to me. What I have heard is grrm wasn't involved after season 4. Also HBO offered more seasons to do what ever they needed. The two writers decided to make the last season short.

My understanding is the show runners were trying to jump ship onto a Star wars project, so they rushed out crap, and then star wars fell through.