If I remember correctly, the SNP ran in 2016 (after the Scottish referendum) with a manifesto that said if UK voted "Leave" in Brexit but Scotland voted "Remain

If I remember correctly, the SNP ran in 2016 (after the Scottish referendum) with a manifesto that said if UK voted "Leave" in Brexit but Scotland voted "Remain", they would push for a second independence referendum. So this was the platform on which they were returned to government. Also, I think it's fair to say that Britain leaving the EU is a sizeable enough change to counter the "once in a generation" line — the choice is no longer Independence V status quo of 2014. Especially considering that at least some people voted to stay in the UK in order to remain in the EU.