I have a similar problem to the one under this question: What is tumblerd?
That program is one of the most CPU hungry I ever seen in Xubuntu/Xfce. I do not really need thumbnails preview and I might just uninstall it.
sudo apt-get remove tumbler
But I get:
The following packages will be REMOVED: tumbler xubuntu-desktop
Isn't that my desktop?
EDIT AFTER SOLVING THE MEMORY PROBLEM:
I would like to know the answer to this question, but considering the initial reason for removing tumbler: removing tumbler is not necessary.
The sudden memory use is related to video thumbnails, and that only for videos that are moved, downloaded etc.
To completely disable thumbnails, open a Thunar window, Edit-Preferences-Show thumbnails:Never.
To disable them for video files only, edit
/etc/xdg/tumbler/tumbler.rc changing the lines
Disabled=true under 'Video Thumbnailers' section.