This is a similar problem to something I experienced my first time with KDE on Mandriva, although it was much milder this time.
I have a smartphone which receives email via IMAP. I subscribe to this same IMAP account via Kmail as well. I had a new email in Kmail which was marked as unread, but which I had actually read moments earlier on my smartphone. I clicked on the email in Kmail, and boom, all of a sudden, all of the emails in my inbox started "scrolling." I pkilled kmail and restarted it, and the problem went away. I have since set "mark emails read" in kmail settings to delay for 5 seconds; hopefully that will solve things for now. My intuition is that kmail saw the email as unread, and when it opened it and found it was already read, it freaked out.