If this is all a hoax

There's no "if" about it.

HOW did it start and why THIS DATE.

Numerous individuals have made numerous claims, and the Internet has facilitated the easy spread of these claims. If you're interested in specifics, this site covers them in detail.

More and more hoaxers have started trying to shoehorn their own pet doomsdays into 2012 (e.g. Nibiru), as the subject has become more popular over the last few years. You can read about several of the more prominent doomsayers and mystics by consulting the list in the main menu to the left, under "Proponents."

As for why December 21, a cycle of the Mayan long-count calender ends on the winter solstice of what we call 2012. It has nothing to do with Islam. Moreover, any doomsday claim, including that in the Qur'an, has only 7 days to work with, which makes any particular day and coincidences between days in different claims reasonably likely.