It appears to be some sort of "Auto-generated backup commit"
This is a GitHub for Windows hack that (more or less) auto-save your working directory. This feature greatly help their support team when a git newcomer messes up his repo. Thanks to those backup commits, the support team is able to "recover" their uncommitted work.
Is accumulating dangling commits and blobs expected behavior?
Yes, that's an expected git behavior. Every time you unstage a file, or partially update one, this leads to dangling blobs. Every time you rebase a branch, or rewrite your history, this leads to dangling commits.
This is part of what makes git "safe-by-default". Everything is persisted, and through the reflog (or the output of git fsck) one is able to access those previous states/snapshots.
Those dangling objects will be automatically cleaned up according to you config settings (see git gc documentation) or the next time you'll the git gc
command.