Brexit became a possibility the second UKIP became a threat to the electoral chances of the Conservative party. It's very likely that Labour would have won in 2015 if Cameron didn't promise that referendum.

Scottish Labour already has a committed anti-independence position, so they likely will let the SNP win over doing a similar promise. The only other possibility is a coalition government with Labour, but I don't see any government making the same mistake after 2016.

Ha yeah. Honestly though if Nicola Sturgeon had the sort of control of Scottish politics (and "popularity") that Putin does in Russia or Orban in Hungary, then some kind of escalation could have been on the table. But I imagine she knows that making any kind of move like that would be political suicide and would kill off a lot of the hard-won support for independence that they've grown over the years.