Eh, I think it was a combination of things.
I get that more than a fair chunk of this is my own conjecture, but I think it's...mostly true.
JTC started out as an all-out grinder, threw himself in front of every shot, also scrapped like hell in the offensive zone. He also got hurt a lot. Things went completely south after Kerfoot was dealt and he was tabbed as the 3C going forward. He knew he couldn't play the way he was before and not risk further injury so he got hesitant and damn near played his way out of the NHL. He found his way a bit when they stuck him back on the wing but he continued to struggle when they put him back at center.
Broken record time--I still think it all changed once Helm arrived. That guy taught JTC how to be effective without necessarily selling out physically, especially on the penalty kill. I know there's a fairly strong argument that it kicked into gear the moment the team finally dealt Tyson Jost, but IMO we were seeing him start to "get it" well before that happened.
IMO the new contract and the bottom falling out of his game were a bit of a coincidence--they threw him into a role he simply wasn't ready for, and the things that got him to the NHL in the first place were kinda/sorta leading to too much injury and he couldn't immediately figure out how to balance it.
Again, I still don't think a long-term market-value deal is a good idea, especially for Colorado, but I hope he gets his $$$ regardless.