It's really simple, for the first time in a generation the referendum got people up here interested in politics and the theory (yeah I know) is that you can shape your future, we are engaged in the process if you like. The SNP has become a movement much like Labour was 100 years ago and we seen what they have done. Their policies in Scotland are also attractive to people so largely they play their hand very well.
One key to the current surge is that Alex Salmond is no longer Party Leader and I believe that should all current MPs be returned he wouldn't be the leader of the party in Westminster either. Not only are they preaching progressive politics but in not having Salmond as a party leader in Westminster the Party is progressing too, He wasn't particularly popular as a person.
Key problem, as has been known for years if you do your research, is that Scottish labour are just the thread of a party being controlled from Westminster, their focus is elsewhere and they not seen to do anything positive in Scotland. Their emphasis in Scotland has changed since the loss of all the heavy industry and the tight control of the unions within the shipyards, steel mills and car plants of the central belt. I would not be shocked if they are wiped out but surprised if they gain 10 seats or more.
Put it this way if the turnout in Scotland on the 7th of May approaches that of the referendum, Scottish Labour and the Scottish LibDems could disappear from Westminster altogether.