So here's the scenario - the Conservatives get 30% of the vote and 280 seats Labour get 31% and 260 seats, the SNP 50 seats and UKIP (15), Lib Dems (25) and Northern Ireland (18) and others get 60 between them. (plus the speaker)
People notice that Labour got more votes than the conservatives but fewer seats, UKIP and the Greens got more votes than the Lib Dem and SNP but fewer seats. Nearly 40% of people didn't vote Labour or Conservative yet they have less than 40 seats in England and Wales out of 572.
The Conservatives as the party with the largest number of MPs forms a minority Government, the 70% who didn't vote for them almost 85% of the electorate) get mightily pissed off. The new Labour Leader, stuffed by FPTP in Scotland, claiming an moral election victory because of winning more votes for the Tories and despairing of winning a majority announces support for electoral reform. Suddenly leaders of parties who got 70% of the vote want electoral reform. Alternate Vote has already been rejected so a proper proportional system is the only option.