this just happened to me the other day (could be around the same time this was originally posted?). it basically kicked me back to the login screen. i just assumed that something kde-specific was being updated. but it was the first and only time that i've seen this happen.

Never happened to me on Arch (Antergos), it never crashed as to lose my session, but for a long time after update of plasma sometimes the shutdown countdown did stop working so I could not restart kde from UI, had to use terminal.