I disagree and for 2 reasons:
Firstly, the SNP were threatening to derail the Tories from very early on in the campaign, the SNP prediction of seats was well-known early on. If people were scared of Lab/SNP then they would have started switching at that point. The polls didn't indicate that...
I think it far more likely from the start that the opinion polls were fundamentally wrong with some aspect of the algorithm that they use. I don't recall massive swings away from Labour to the Tories from them in the very last days before the election. If the fear factor had crept in, in he last...