This is due to the CPU peaking at 100% on the same core and the management process (winbox) causing Winbox to timeout.
You will need to re-login with Winbox or try a SSH session instead.
Yes that was my guess for the logoff reason, but the problem is, during massive BGP feed import I can relogin 10s of times and each time I got logoff right in 2-3 seconds. This is weird both for admin purposes (I can not see what's on on the router during minutes depending on feed import speed), and as an attack surface, as if someone is able to maintain 100% CPU level for minutes and keep admin off the device the he can control the network stability.
I do hope to 7.1 to keep up this but really wonder if I can limit BGP process to use less that 100% CPU or any other way to see my device all the time.