It's only gone over 50% because of Johnson
It has been on the rise since the referendum for a few reasons.
First things was after Cameron made all these promises and then as soon as they voted no the conversation turned almost instantly onto the effect on England.
Brexit also had an effect, voting Remain yet the UK as a whole voted to leave. Their voice once again overridden by England
Then there's Johnson who despite claiming to be a one-nation Tory has done more to assist the breakup of the union than anyone before him with his Irish border plans and just general impression of England being the 'important' member who's needs much override everyone elses.
Plus his catastrophic handling of corona whereby Sturgeon and the other home nations began to openly step away from UK advice because it was so unclear and didn't seem to follow the science but economic ideology. They've not done a bad job of giving out their own advice to a certain extent and has emboldened the independence voice by showing we can manage ourselves whereas having to listen to those in Westminster is actively detrimental.
So it may have gone over 50% UNDER Johnson, and his mismanagement and attitude has been a big factor in it, but it started gaining further momentum long before Johnson. Had they had another vote post-Brexit while May was in charge I reckon we'd have still seen a yes vote. The only change under Johnson has been it being a probable yes to an almost definite yes with the possibility of a landslide in favour.