I am surprised that no one mentioned the theory of Nemesis? Oh wait, there was a Slashdot story (http://science.slashdot.org/article.pl?sid=06/01/21/0338234) on it, or at least on the possibility.
According to this site (http://www.bibliotecapleyades.net/ciencia/ciencia_nemesis07.htm) :
"Nemesis' existence was proposed in 1984 by American physicists Daniel Whitmire and Albert Jackson (The University of Southern Louisiana) and also by Richard Muller, Piet Hut & Davis M. in 1984 in order to explain an apparent 26-million year cycle in the occurrence of mass extinctions on Earth, like the one that killed the dinosaurs 65 million years ago, as noted by Raup and Sepkoski."
So, simplified, the other sun rotates in and drags a whole bunch of crap out of the Oort cloud and wipes out (almost) everything. Lather, rinse, repeat.