Majority of the people voted to stay together in the 2014 referendum. Why is a second referendum proposed? One theory I have is that it might have something to do with brexit, but I’m looking for something more concrete.
The 2014 independence referendum was accepted as being a once in a generation referendum, unless there was a "material change of circumstances". The SNP is arguing that Brexit amounts to a material change in circumstances and therefore the criteria have been met for another referendum.
One of the big issues in the 2014 referendum was Scotland's membership of the EU. Scotland is generally in favour of the EU, and during the campaign many pro-union people made a big deal of an independent Scotland's chances of joining the EU, saying that the only way for Scotland to be a member of the EU would be as part of the UK.
However less than 2 years later the UK government announced there would be a referendum on whether the UK should continue to be a member of the EU. Scotland voted heavily in favour of remaining in the EU (62%) while the UK as a whole narrowly voted to leave (51.9%). So Scotland was now being removed from the EU against its will.
There's no way of knowing how much the EU membership question affected the 2014 referendum results, but the SNP argue that leaving the EU alone represents a significant change in circumstances. They also argue that the UK government's approach to Brexit is detrimental to the interests of Scotland. They say that the UK government has not included the devolved governments of the various parts of the UK in discussions, and that their approach amounts to a power grab.