Mail coming to fore window for no reason?

Having this odd problem since getting Mojave. I could be sitting there at my desk and the Mail app pops up on the screen with no mail in it, by itself, no clicking or typing. When it happens Mail is running in the background. After I check my mail when I first turn on my machine I click the red tab to close the window and let mail run in the background (active in dock). Weird issue, anyone else experiencing? Happens on Mac mini and Mac Book, so pretty sure it's an MacOS thing.