I'm inclined to suspect that the wheel spacer was a victim of the problem, rather than the cause.

If the spacer was properly fitted & the wheel nuts done up securely, then it will all stay together, but wheel nuts that are not secure, ... well that's a different story.
This is by far the most common cause of wheel sheding.

The only other thing would be if the spacer broke up & pieces fell out allowing the wheel nuts to be damn loose, which seems unlikely.
I suggest that you see if you can find the missing pieces of the spacer & note their location. If it's a bit of a way back before the location of the nuts, then this would be the cause, but if at the same location, or after the wheel came off, then unsecured nuts would be the main suspect.