The design changeover is being driven from the top. Ever since Steve Jobs has died and Larry took over as CEO, he's gotten the design religion, and his goal is for Google's design to remain fresh and drive trends forward perpetually. So as far as the company is concerned, this is a feature, not a bug.
It's true that the individual designers responsible for doing the design often vary from project to project. However, there's a fair amount of continuity as well. The designer who initiated the design refresh announced today has been with the company since 2006; the designer I worked with for the visual refresh of 2010 now heads up design for all of Search. They are explicitly told by executives to make things fresh and remove previous constraints when imagining the new Google.
Is this the root cause of why Google Maps/Nav on Android had a giant UX regression from 6.x to 7.x and still sucks so bad that my next phone may very well be a Lumia?
Snark aside, I think you're seeing two effects. One is designers wanting to be creative and innovative (which is a top-down directive) without also talking with their counterparts in other areas of the company. This will get corrected over time; periodically the company tries to line up all of its products so that they're consistent across all of Google. The last major such project was Kennedy; Quantum is the next one, so I suspect Android G+ will eventually change to conform with the Quantum styleguides just announced.
The other effect is that design rules are different for big companies than they are for small app developers. Small app developers want to fit in with the platform styleguides, because they face intense competition, are only a tiny part of the total ecosystem, and so if they deviate from the expected UI it only harms themselves. Big companies want to set the standards, and so they encourage their designers to be bold and adventurous, in the hopes of creating the trends. And the Android G+ app has been a trendsetter in the past; some of the recent move towards very image-heavy apps (across both web and mobile) was pioneered by them back around 2011 and 2012.
presumably the week before the next UI paradigm is announced at I/O 2016...
Honeycomb was introduced in 2011. That's three years.
And besides, Android is on design iteration #3, Apple is on #2, and I think most would agree iOS was aged pretty badly by the time it got #2.
Personally, I find Android's 2.x design about as much as a design as a factory floor's sounds 'music'.
What does this even mean? It reminds me of the design-by-committee meeting to add Poochie to Itchy and Scratchy. "It needs to be more dynamic. More proactive. More ATTITUDE!"