I don't think there's one correct answer to what happened with the trade, there are multiple factors that have been mentioned on these threads over and over which you can feel free to weight depending on your opinion:
Russ was tired of Pete's tired coaching philosophy
Russ isn't as fast as he was ten years ago, and it shows. Plus he's probably one of the most studied QBs playing, so if PC won't let him try anything new he's going to get hammered by predictable play calling.
Russ was tired of getting hit/pressured. When your coach won't listen to you, and you keep seeing the same failure over and over, it kills your spirit. We have all probably worked at a job like this.
Russ has too many off-field distractions - when you succeed at his level, you think you can do all other sorts of things and it won't get in the way. This has been the downfall of many men.
It only works to give your QB the big bag of money if you let him have input, or is on the same page as the coaching and GM staffs
PC and JS didn't cut bait soon enough with their winning formula from 10 years ago, resulting in them painting themselves into a draft/FA corner because they wanted to stick to a style that RW was no longer interested in/ceased working about 4-5 years ago. (see above about paying a bucket of money to the most important player who has diff ideas than the coach)
Pete fell into the old Big Boss trap of wanting your middle manager (QB) to step up more, but not letting him make any decisions on the fly without sign off from Big Boss, which is always a recipe for failure. it's called delegation of tasks, Pete, look it up. All NFL coaches are micro-managers of course.
Instead of acknowledging that their tried and true no longer worked, PC and JS tried more than a few seasons of stop gap measures by plugging in obvious weak spots to stop the flood, when it would have been better to 'retool' certain areas of the game.
And lastly: QBs are getting way overpaid in short term winner-take-all seasons, instead of actually developing overall talent because EVERYBODY WANTS RESULTS YESTERDAY.