I just spastically double-click words all over the page while I'm reading. No real rhyme or reason to it, it doesn't help me read or anything. It's mostly just an unconscious tick.
I too do this, and sometimes compulsively drag large portions of highlighted text around as well. Annoyingly, Opera seems to open in new tabs any links in the highlighted area when the dragged text is released. This has led to me up/down voting everything at various sites on more than a few occasions (but ah, they shouldn't have GETs do anything anyway, I say :)).
Drives my boss crazy.