Many factors but probably the biggest is the parade of Conservative governments since and including Margaret Thatcher.

The Conservatives (or 'the Tories') have been wildly popular south of the border but not so much up here. Scotland was very much Tory country at one point but that changed with Thatcher. Her economic policies absolutely devastated many of the poorer parts of the UK but Glasgow and the west of the country were arguably the worst hit. Glasgow has come up a lot but the deprivation there compared to places like London is obscene. Until recently it was one of the poorest areas in Europe (its possible it still is).

Thatcher was followed by Cameron (caused Brexit and 'austerity'), Johnson (Brexit mouthpiece, covid spending), and Truss (50 days in power and managed to crash the economy) amongst others.

The way Westminster works is that every MP is supposed to have roughly a similar number of constituents. Because we have a smaller population than England we have far fewer MPs. So typically the SNP or Labour clears the board in Scotland, while the Tories take England. This leads to a feeling that the Tories are 'imposed' on Scotland as a consequence of the union. Thus, if we were independent we'd be rid of them.

There's other factors, like sectarianism (protestant unionism vs separatist Catholics), wealthy (ie conservative) landowners, etc but the Tories is where most of it comes from.

The right... farming and fishing. There is (or used to be) a feeling that the Tories looked after the farmers and fishermen. Thats largely been trashed since Brexit (both groups lost lots of European grant money). The wealthy landowners have always exercised their power via the Tories and the print media - they worry their land will be taken off them in an independent Scotland (highly likely). Then theres that old chestnut of foreign influence via social media, exacerbating the immigration debate. The right is predominantly Unionist, thus if you're right-leaning you're typically unionist.