Anderson fell off a cliff, post-surgery Karlsson (I wonder in hindsight if it would have been best if he took another month before returning), potential disconnect between players and coach. It doesn't really take much to go from getting into the playoffs by four points to having a disaster bottom five season.
I would rank it like that :
1) Karlsson post injury, really struggling defensively and in transition. Not having the same impact offensively.
2) Anderson fell off a cliff (due to age and riding an emotional high last year), Condon proving he is not that good.
3) Methot lost to expansion, Claesson and/or Oduya can't fill his shoes. Left a really big hole on the left side, no #2 D-man, exposing Phaneuf and Ceci who are #4
4) Boucher lost it, his system clearly not working anymore : team defense in shambles. Coaching staff can't adjust. Oh and Dave Sparky Allison made a good point, special teams are terrible this year.
5) Turris trade : clearly affected the locker room. Even though Duchene is technically a better hockey player, a trade like is not good to make at the start of a season
6) Most Pejorative Slured schedule ever
5) MacArthur was still on the payroll, expected to be part of the team, your #6 forward gone for good
6) Ryan who was still the best #4-5 forward keep getting injured. Ryan-Brassard-Stone was a really good line before his first injury.