With the advent of Windows Vista there are changes made in how the operating system determines if it can take a kernel memory dump or not. Starting in Vista the amount of memory allocated for kernel mode could vary dynamically. If the pagefile is not big enough, switching to minidump at dump time can’t be done easily. So the dump stack initialization is happening at the time of boot where this check for the pagefile size is done.
What does this mean? It means if you don't have a pagefile as large as physical memory at boot, and your system is configured for a kernel dump, you'll end up getting a minidump. If you permit me to opine, this makes sense in the client space where a valid dump is more critical than a corrupted kernel dump, as the results usually would get uploaded to Microsoft via WERCON or another mechanism. If further triage is needed MSFT could contact you with the ability to setup a kernel capture.