The defeat of Eric Cantor, the House Majority Leader in his Congressional primary in Virginia by David Brat, was stunning and shocking, but not the first time that a Congressional leader was defeated for renomination or reelection.
Other prominent cases include:
Senator Richard Lugar, former Senate Foreign Relations Committee Chairman, of Indiana in 2012 by Richard Mourdock
Senate Minority Leader Tom Daschle of South Dakota in 2004 by John Thune
House Speaker Tom Foley of Washington State in 1994 by George Nethercutt
Former House Ways and Means Committee Chairman Dan Rostenkowski of Illinois in 1994 by Michael Flanagan
Congressman Emanuel Celler, House Judiciary Committee Chairman of New York, in 1972 by Elizabeth Holtzman
Senate Majority Leader Scott Lucas of Illinois in 1950 by Everett Dirksen
Senator Robert La Follette, Jr of Wisconsin in 1946 by Joseph McCarthy
Notice that ONLY Cantor, Lugar and La Follette, Jr. were Republican officeholders defeated, while the others listed were all Democrats.
So it is newsworthy and highly unusual for Congressional leaders and prominent members to lose reelection!