Switching between applications always hides other apps When switching between applications how does one allow applications to stack or remain in view? 

Example: I have a web browser open (not in full screen but sized to cover the full display) I then open the calendar which is sized smaller and used to appear in front of the browser in previous OS versions whilst still displaying the web browser behind it. 

What occurs for me is that when I switch applications the previous application is hidden to the dock, not minimized but hidden.

Here is a short gif as I have most likely explained my problem terribly.
Do you have Keyboard Maestro installed? Have you accidentally selected this option?

                                  

The slight delay between clicking an app and the previous app being hidden (shown best when clicking an app with a smaller window after a larger window, exemplified in the frame below) indicates that this is not the system performing this action but rather a separate app detecting the trigger and acting accordingly (hence my suggestion).