speaker volume issue

Are you only using the Sonos app for control or do you have other controls by other services?

Is this a new setup, or is it a change of behaviour in an established system? If the latter, it may be a fault in a speaker. 
Generate a diagnostics report and speak to Sonos support. 

I never think about the Play 5, Five and Era-300s being aware of their orientation, interesting to note the 300's reaction.

Interesting that the Era 300's orientation affected the volume behavior. I wouldn't have expected the placement itself to cause the volume to change automatically, so that's a useful detail for anyone troubleshooting a similar issue.

It might be worth checking whether the speaker's orientation detection is also affecting other settings or playback behavior.

As the speaker was upside down, I would think it more likely that the volume buttons were touching the surface and the Era 300 interpreted that as a “volume up/down” input. It does seem odd though to want to use it with no access to the controls and not on the stable base they have on the (normal) underside.