Apple Watch (latest watchOS) switches faces while idle

My Series 1 watch switches faces without intervention. Best example happened some hours ago this morning. I got it at about 3:00 am for the usual reason; I checked my watch which was display the time in the analog Utility face. When I got up later, the face was a digital display of London/Big Ben.

Why does this happen? When I do an update or power OFF/ON the watch, the face is always reset.

Addendum: while typing this post, the face changed to Activity!