The SNP haven't done so well at Westminster in the past, Scotland has been dominated by Labour for decades. However, they were in minority gvt in the Scottish Parliament from 2007, and now command a majority (under a system specifically designed to prevent majority gvts) there, and they're proving popular. The Scots were promised all sorts of extra powers if we voted No, and that's what we did. Instantly, the offers vanished; atm we'll have the right to design our own roadsigns, and that's it.
Many Scots, whether they voted Yes or No, are a bit narked about this. They/we feel that since the British establishment threw everything at us asking us to stay in the union it's a bit much that they'd like us to STFU now. I don't think anyone expected it, but that "bit narked" looks set to translate into many, many more seats than the SNP have ever held, if the polls are correct.
And that's why we're seeing so many horror stories about how awful the SNP are, and why MPs from Scottish constituencies shouldn't have any say... They're really not all that dreadful.