After some CPU trouble with Nepomuk viewtopic.php?f=225&t=108075#p254296 I ran into some other CPU trouble.
For no reason that I'm aware of, Dolphin didn't respond to anything and instead turned grey, not showing any folders/files anymore.
I looked at "top" in terminal and dolphin was using 100% cpu.
I clicked the "x" in top right corner a few times to try and close it and got this message:

The window "8824-F862 – Dolphin" is not responding. It belongs to the application dolphin (Process ID = 3724, hostname = localhost).

Do you wish to terminate the application process including all of its child windows?
Any unsaved data will be lost.

After killing it the CPU stayed higher than normal and in "top" now there was a 100% for "kactivitymanage".

Any idea what's happening and what to do about it?

"kactivitymanage" is a shortening by 'top' of the full name 'kactivitymanagerd' which is responsible for co-ordinating Activity related information from applications and the Plasma Desktop. It uses Nepomuk to achieve part of this goal. It is probable that both Dolphin and kactivitymanagerd began using 100% of your CPU due to Nepomuk related problems.

Thank you for the clarification