We all know that the fearsome dinosaurs (and a host of other creatures) perished when a large meteor (size of a small city) slammed into Earth some 65 million years ago. The impact crater is widely believed to be the Chicxulub Crater, near the present-day village of Chicxulub on the Yucatan Peninsula. Scientists (Richard Muller and his graduate student, Robert Rohde, from UC Berkeley) are now reporting that such extinctions took place with surprising regularity. A painstaking computer model, going back 500 million years, suggests that (about) 70% of the earth species are destroyed every 62 million years or so.

Chicxulub Crater (Courtesy: UC Berkeley)The researchers analyzed data that cover the first and last known appearances of no fewer than 36,380 separate marine genera, including millions of species that once thrived in the world's seas, disappeared, and in many cases returned.
The explanations might include quirks of Solar Dynamics (some hidden solar cycle), a companion star to our sun (which might occassionally nudge the asteroids in the Oort Cloud), or even the solar system passing through the Galactic Plane (resulting in a higher chance of comet/asteroid impact). There is also the possibility of a Earth cycle that perhaps triggers volcanoes/earthquakes or other natural disasters.
Are we already due for another major extinction? Scientists do not know yet. It might be a long drawn phenomenon, or a quick culling. Take your pick.