It goes back millennia. They'd raid us, we'd raid them. Conquer them, then they'd boot us out and take Northumbria. Got down as far as York once, as I recall my dim and distant history lessons. So there is a long history of animosity.
As far as this goes though, it started about 50 years ago with the formation of the SNP (the Scottish Nationalist Party). They worked on two planks - Scotland for the Scots, and national pride. When devolution happened and Scotland and Wales got their own Parliaments, the SNP got into power pretty quickly and has been pushing for a referendum ever since. Last year, the SNP and it's allies got enough votes to set the date.
They certainly are not conservative! Every single MP in the main Houses of Parliament in London is Labour, and pretty far left, at that.