I think it's a mixture of Tories capitalising on England's fears about an SNP/Labour coalition, together with the fact that Labour have/had a very weak leader. But the ball started rolling when Salmond said he would be writing the Labour budget!
It's 1979 all over again, the SNP helping deliver a majority Tory government. Expect Cameron to offer devo max pretty quickly.
There's also a rise in UKIP voters - 3.5 million I think (twice as much as SNP?)! Only good thing about FPTP was keeping their seats to a bare minimum.
As for the Lib Dems, it's a genuine shame for Charles Kennedy IMHO. Anti-coalition, anti-war, and a very decent bloke and good constituency MP.