I was doing great as Carthaginians with only having declared war with Babylonians to get my Bireme home (since he refused open borders and Bireme was slowing losing health). 

When I switched over to Medieval as the Norsemen it made me a vassal to that Empire without even prompting me. And yes I was able to recreate the issue by reloading an auto save.

https://imgur.com/a/SiiYiqB