I love this turn of events. I didn't think a season 6 would be possible because the season 5 finale apparently coincides with Infinity War and the next two films take place before IW. We don't know how much time takes place or what will occurred in Avengers 4 so I doubt they had much wiggle room. I don't think moving it to summer was ABC's call, I bet Marvel or the showrunners requested that and as a result ABC wanted fewer episodes. I would guess their reasoning for a shorter season is so that if the want to renew for potential season 7 and 8 they could order another 13 to premiere in Winter 2020 then go back to a fall release later that year. Still I love the idea of waiting till post Avengers 4, I love having a shorter more focused season, and I love giving the writers time to breath and reassess. It's happened plenty of times for a variety of reasons. There's no real rule towards it, just whatever the studio agrees on for whatever reason. Whether it's so a start would be able to do another project without leaving the show, make room for a different show, go on hiatus for some unforeseen circumstance. It's somewhat unusually but it's happened. Also season 4 was cut down significantly because of the writer's strike I don't think Dethlok can really hold his own show. I always thought that Ghost Rider would make a great show, have it star Johnny Blaze, Robbie Reyes, and a third Rider each following their own paths that leads to each other. Think Supernatural.