Both my virtual machines on my cloudservice were restarted sequentially without warning (at 3.57 gmt) (and certainly not triggered from within the machines according to the system logs). This is exactly the same behaviour as occurs during planned maintenance which makes me suspect that this was a maintenance operation. During maintenance operations I boot up extra machines in the same availability set, but as I didn't know this was going to happen, my site and all the sites i host were offline for 20 minutes.
Should I expect random restarts like this in the future? Running an additional machine in my availability set at all times is expensive!