I have acquired this habit from the 2 years I've spent playing Starcraft : Brood War and Starcraft 2 semi-competitively.
In the game you have to select units by dragging a box around them, and that habit has carried over to selecting text on the screen in pretty much every computer application.
I know that probably doesn't answer your question, but that would probably require a neuroscientist. It doesn't help me read personally, it just seems to be one of those repetitive tasks that slightly autistic people do.
Some of my coworkers share this habit as well, and they've never played RTS games, so it's acquired in different ways.
I do not have any data on it (1) but I think it's like moving a bookmark across the page as you are reading: it makes it easier to jump from the end of one line to the start of the next one.
(1) not even on the claim that that bookmark has that effect.
In french I say "astigmate". Either vertical or horizontal lines are blurred. Plus maybe you're tired. Anyway you can follow straight lines and you select something to add a visual clue of your position. "It's all because of" modern designers and their featureless screens.
