You're right, that is a weird one! Not heard of that happening before.
I'd suggest something has gone awry in your session settings somewhere. I'd start by creating a new user account, and logging into it. Hopefully the problem won't happen for this new user, so in that you'll know that there's something wrong in your user config for your main account.
If that's the case, make backups, and then delete the KDE config files from your main user account.
If the same problem does happen in the new user account, without you making any changes to the config, then you may have some weird installation glitch. Honestly, I don't know what to suggest in that case, since I've not come across this sort of bug before.
But let us know how you get on. :-)
All the best,