mdworker (visible in Activity Monitor) is the Spotlight indexing process (active even though the process is not visible in the spotlight menu).

We had a situation where movie files from a mobile phone (file ending .3gp) caused problems, as the indexer didn't seem to get past them. Using up 99% and more percent CPU which caused the fan to go crazy. Removing the files resolved this CPU madness.

You may find which files could be the problem if in Activity Monitor you click on the mdworker process and the click "inspect" in the menu, then choose "Open Files and Ports".