What is tumblerd?
Solution 1:
It's another program called tumbler that is part of the XFCE standard installation (package tumbler).
From the package description:
Tumbler is a D-Bus service for applications to request thumbnails for various URI schemes and MIME types. It is an implementation of the thumbnail management D-Bus specification described on http://live.gnome.org/ThumbnailerSpec.
Solution 2:
(followed Daenyth's suggestion and moved this from a comment to an answer)
tumblerd may be eating lots of CPU if you:
are downloading some huge media file (e.g. movie) and
have the download folder open in Thunar.
I'm guessing every time the file grows a tiny bit, tumblerd will check to see if it has changed, and try to remake the thumbnail. Closing the folder makes tumblerd stop.
There's a XFCE Bugzilla bug report on it, still open as of yet. See also the list of open tumblerd bugs.
(Apparantly it may even happen with completed downloads, though this may have been fixed in 12.04; I haven't confirmed it myself).
Solution 3:
The bug is still present in Xubuntu 14.04 x64.
The workaround of editing /etc/xdg/tumbler/tumbler.rc and disabling video file thumbnailing works fine for me. Image file thumbnails are more important to me, and those work without problems.