It's your browser, not PC, that opens tabs (or windows) and takes you to them (or waits till you click on the tab yourself).

In the browsers that I've used the most, Firefox 2 and 3, there are settings for this under Tools > Options > Tabs. Other browsers probably have something similar but I couldn't tell you how to get to it.

It's possible that you pressed some keystroke combination you weren't aware of, that your browser settings got changed somehow, and/or that your computer hiccupped. Occasionally when a page is still loading, the processor is busy and there's a long lag, I'll click on a link where I see it but by the time my browser "sees" the click, the page has changed under it and I find myself being taken someplace I hadn't intended to go.

Hope this helps, or at least points you in the right direction.