Also, the tsunami and I'm thinking the B2W2 announcement (staff was likely told in advance) really screwed up the plot the writers had going. It wasn't terrible, I think it just bit off way more than it could chew.
Now, you are into something.
People keep saying "it was the tsunami™!!!1!!!!!" (and, unless I'm misremembering it was an
earthquake at the time) but it probably wasn't that because episodes are scripted and planned in advance thus aside from episodes being postponned "indefinitely" (TP vs TR) and some messing around with episode order, things were more or less on the rhytim they were supposed to be.
What probably screwed them up was the whole thing with BW2 since the changes it brought, specially on the Team Plasma front really screwed up everything they planned doing with it. So I'd say it was less about very unfortunate real-life events and more about GF suddenly deciding to go bold and do something new and surprising thus screwing everything the anime's plans.
On a minor scale, I'd say the backlash over the changes on the TRio played something. It's either the thought that it would lead to a backlash or it not working as they inteded. Maybe issues with the VAs since they appear to have not been content with it.
Tomiyasu's version of Ash in general tends to treat him more like a child than ever before, while all others treat him as more of an adult, so it makes it really tiring to hear this excuse used to explain away SM's problems, when it really doesn't.
I wouldn't use "adult" as I don't think it fits. I wouldn't even use "mature" as some people (not you, by the way) like to say either.
I'd say more as the leader of the team or at least as a kid who learned a few things and has to help his friends. He's still a kid for all that matter, it was just the writting focusing on his role model/ideal archtype while SM went with the "crazy for Pokémon", "hot blooded hero", "excited kid" angle. Both valid and fitting Ash's characterization across the years.