It may help us to know the basics of your computer system (graphics, driver, etc). In the meantime, have you run SolidWorks Rx diagnostics to see if it flags any problems?
SW 2013sp5.0 (also installed 2010-2014)
I have not recently run Rx, but have in the past with no reported issues...
Referring back to my earlier speculation, I may have made an association with Firefox...?
Update; I just had it happen without leaving SW. I did notice that it happened when, on my Logitech mouse which has a "tilting" scroll wheel, I inadvertantly depressed AND "tilted" the scroll wheel to the side...
Updated (again): This is odd behavior, possibly a mult-point failure? When it does happen, sometimes, when I click in an open Firefox window, it seems to correct itself. Could it be just having Firefox open causes it to happen, without even having it as an active window?