It's really a business problem. Once you have a successful high margin product, investors expect you to continue making those numbers. If you sit on your duff then competitors will catch up and you find yourself in a low margin commodity market.
But is it isn't always obvious what to develop, especially once the low hanging fruit is gone. Sometimes there isn't a right answer and the market is simply mature enough that it's destined to become a commodity market.
And if you can't find something good to do, people have an aversion to doing nothing. So instead they do something bad.
I wish Apple had instead focused, for the whole year, on getting iCloud Photos 100% right. Unlike "flat" redesigns, this could even have turned into passive income for Apple. It hasn't even shipped on OS X and I've already given up on it. If they'd just frozen OS X at 10.9, everyone could still work on cloud functionality for years before they'd be "finished".
Oh yeah, these pressures are both internal and external. There is a reason concepts like NIH, bike-shedding and "reinventing the wheel" are endemic problems in the software world. I guess we're just seeing that Apple is not immune from them, after all.
My guess the other problem is simply this -- they hired so many developers, designers, managers. They produced a great stable OS, worked hard, then it was making billions of dollars. Ok, what next? Institute 4 day weeks? (maybe not a bad idea, but I am being hyperbolic here a bit).
So there is perhaps some pressure from the inside to develop new features. Designers say "flat is best now" so they get to work and feel busy. Developers want to build the next internal restructuring. Managers probably want to be responsible for some new awesome feature and seal their legacy.