It was reported both in CloudDrive and in the system event logs, so I'm now assuming the system was actually crashing while shutting down, and CloudDrive was doing what it was supposed to by recovering. Unfortunately CCleaner has since removed any dump files, so I'm not able to investigate any further at the moment.
As tends to happen with these things, I've been unable to reproduce it since. I've excluded system memory dumps from CCleaner in the future, so if/when it happens in the future, I should have more to work with - it could well just be a wobbly driver or something.