The issue is quite simple to reproduce: open your qtwebkit browser (QupZilla, Arora, Rekonq, Konqueror, doesn't matter), load some longer video from YouTube (but previously configure YouTube for HTML5: www.youtube.com/html5) and close the tab. Watch your network activity and you will notice the video continues to load in background until it is fully loaded or until you quit your browser.
I haven't able to reproduce this with Qt5.
carewolf on IRC suspects it might have something to do with gstreamer in webkit.
Another example page where this happens is here:
*** This bug has been marked as a duplicate of bug 109350 ***
Possible work-around: Call setHtml("") before closing the tab.