No, it is not a bug. Rotate your head 180 degree vertically. It should read “Upside Down Writing to be a Fun Tool.”
People are always looking for new and fun way of impressing someone and making themselves different from the rest of crowd.
Upside Down Writing became popular when they launched a groups on facebook. Word of mouth accelerated the popularity of the unique feature and it sent the original website so much more traffic.
Just go to their website and you will find your way on your own.
There are several different optional websites you can try. Other than Upside Down Writing, you can try Flip, Rot180, Fliptext.org, Fliptext.net, FlipThisText.com, USD encoding, NQAS, UpsideDown.info, UpsideDownText.com,etc. You can explore a bit and see what you like the most.
With such unique technology, one can become very popular and can anticipate stable business with banner ads as long as it keeps innovating.