William of Walworth I think
Sue,
weepiper,
brogdale and
DotCommunist are all correct. All those factors play a part.
But so does this sort of thing (tweet below, detailing a Daily Mail article saying Scots are raving mad and ungrateful). People see this sort of thing, and their response is "Oh, you think so, do you? Well, we'll see about that" We have an expression here to describe that sort of reaction, it's called being "thrawn". It means something like "contrary, perverse and intractable".
Part of it is that some people are saying "well, we might have voted to stay in the Union, but that doesn't mean we're happy with the way things are done round here. As you're about to find out".
Part of it is that people have woken up to the fact that Labour has been the party running local government in areas of greatest deprivation since the days of Keir Hardy and more than 100 years later, those are still the areas of greatest deprivation. And they're thinking, "well Labour's more than had their chance, and we vote for them time and again in GEs to keep the Tories out, but win or lose, we get more of the same. What's a good way of stirring things up? Oh aye. This".
And part of it is that Labour has just taken their voters in Scotland for granted. In fact, their reaction to being found out has been to insult people.
A commentator likened the phenomenon to the coming of punk. Labour in this analogy is prog rock. Their attitude is entitled, baffled, disgusted and angry. And while the punks (the DIY, self-directed, homemade zine photocopiers intending to vote SNP) might not have all the answers, they have a feeling of release, of wiping the slate clean, and of a simplicity in the way they want to express their rage.
(I think they'll be disappointed, but that's entirely besides the point).
All of this can be seen coming through the indy ref campaign, and with hindsight if you read the indy ref threads (especially the big one in the Scotland forum), you'll see it all there.