The official unofficial explanation from Windows Engineering team on this this is that Windows is pinning GPU Accelerated applications to your primary video card. The engineering team describes it as a "very inefficienct" to use a GPU Accelerated app on a non-primary video card.
They are very dismissive of the fact that this worked for many years and now all GPU rendering applications run a high risk of taking down DWM.
They know of no fix and claim no repro despite the repro being simple. Running multiple video cards in a non-SLI/Crossfire fashion and outputing to multiple monitors. Then moving a semi-active GPU accelerated application to a non-primary video card and monitoring the shared memory GPU allocations. They will spike relative to the activty of the GPU API for the application. Simply scrolling or typing in a GPU accelerated browser (Chrome, IE, Firefox) and you may run the risk of DWM allocating more GPU Shared ram faster than it can release it.
There may be issues surrounding reproing that perhaps are a combination of a ratio of Video Card ram to System Ram, however I can verify the GPU Shared memory spikes on ANY system with multiple video card setup. Whether it takes down DWM.
DWM has a resource usage kill switch that kicks in when it exceeds a resource allocation relative to your machine. I am unsure if systems with more system RAM or systems with more GPU ram are more susceptible as quite often memory allocations are padded relative to your RAM. Start up 10 applications on 3 nearly identical systems but differing RAM amounts and you'll get different memory consumption.
Again the official unofficial response to resolve this is - there is no solution and you should not run multiple video cards in a non-SLI/Crossfire setup.
Disabling GPU Acceleration on the browser also works. IE9 is the only browser I'm aware of where disabling GPU Acceleration results in fairly decreased performance.
Of course this issue also forces one to be very careful of where you run Flash/Silverlight/Movies. On my setup it's pretty much suicide to run any of those on my 2nd video card despite it being the exact same as my primary one.