James Kelly suggests there’s another YouGov poll on the way and warns us it might be bad:

http://scotgoespop.blogspot.com/2020/12/yougov-indy-polling-gossip.html

I had a quick look and found the above, showing that they’ve reported Yes leads of only 1%, and 5% in a period when the average lead across all pollsters has been 7.5%.

This too is of interest:

YouGov was founded in the UK in May 2000 by Stephan Shakespeare and future MP Nadhim Zahawi, at the time both active in the British Conservative Party. In 2001 they engaged BBC political analyst Peter Kellner, who became chairman, and then from 2007–2016, President.

https://en.wikipedia.org/wiki/YouGov#:~:text=YouGov%20was%20founded%20in%20the,from%202007%E2%80%932016%2C%20President.

It’s all down to their sampling bias.