My photos are being deleted from iCloud and devices with no action by me

I was working on my PC and suddenly saw popups from "iCloud Photos" saying "The files have been deleted from all your devices and moved to the Recently Deleted folder on Apple devices. You will have 30 days to recover them". I checked my iPhone and saw that indeed that my photos were all being moved from Photos to the "Recently Deleted Photos" album. I had seen the same thing happen a few weeks ago, but thought I might have inadvertently done something to cause it. However, this time the iphone was just lying on the desk.

There is plenty of unused space in my iCloud+ account. I did notice that there were some iCloud apps installed on my PC, perhaps installed by Apple when I was updating the iTunes app. I never overtly installed iCloud on my PC.

Why am I paying monthly for iCloud+ if I can't trust iCloud storage to be a secure repository for my photos? Any idea why this is happening and what I can do to prevent it from happening again?