Mail app opens randomly in MacOS 10.14.5
This was happening to me for a while, and it became very annoying. I found that if I force closed it and turned the laptop off and on, it worked. If that continues to happen, turn off the WIFI connection and do the same steps. 

As to stop the behaviour in split mode, unchecking the checkbox in mail preferences for "Prefer split view while opening messages in full screen view" helps.