Set a baseball on top of a basket ball and drop them both from waist high, the baseball will shoot really high into the air by stealing energy from the basketball. The same principle can occur when you start breaking pieces off of spacecraft with collisions. Also the two colliding objects might not be in "the same orbit" one could be in a polar orbit and one could be in a more equitorial orbit. One could be circular and the other highly eliptical, etc. This could boost parts of one of them into a higher orbit (though honestly it would be just as likely to slow things down).