1) The polls were wrong
Practically every pollster failed to predict the result. The Conservatives were understated by polls while practically every other party was overstated, especially Labour.
2) The Lib Dems were obliterated
The scale of the Lib Dem collapse far outstripped the party's expectations. The liberals had sight of private polling that showed they would retain their seats: the polling was wrong. Lib Dem MPs didn't benefit from the incumbency factors they had hoped.
3) Ed Balls losing was a huge shock
Probability of Ed Balls winning: 1/33
Bookies failed to clock Ed Balls losing his seat, perhaps assuming that the Shadow Chancellor would get the best pick of Labour's activist resources.
4) Every Lib Dem cabinet minister lost their seat
% of Lib Dem cabinet ministers who lost their seats
The only Liberal Democrat remaining in the Cabinet is Nick Clegg: all of his ministers lost their seats. Vince Cable, Danny Alexander, and Ed Davey all got booted out by their voters.
5) The SNP destroyed Scottish Labour
The SNP recorded the sorts of swings ordinarily only found in by-elections, except across Scotland. Scottish Labour's historical heartland turned pure yellow.
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