It's a phenomenon that has been noticed. And there are a few biological reasons that have been suggested (sorry if any of these are factually incorrect lol, I enjoy reading about this stuff so I learnt a little bit):
Firstly, brain imaging studies have revealed that musicians while they are playing tend to have higher activity in particular areas in the frontal lobe (the lobe of the cerebrum that is concerned mostly with creativity and the deep thinking that humans are known for).
But more importantly and in relation to the link with academic performance, music influences something that a lot of people characterize with intelligence and that's the ability to grasp new ideas, make connections and think of new ideas. The latter can be linked to improvization during music, where the same imaging studies have shown that there is less activity in the same areas. When there is less activity, there is in fact more room for ideas. This is because in a relaxed mode the brain is less likely to ignore stimuli (as it often does when in a focussed mode). So why is music helpful? It's because when improvising a musician has the ability to keep on working and performing at the same time as allowing ideas to flow. If you think about it (well this is how I think about it), when you are doing a hard maths problem it is most beneficial if you can keep on working at the same time as letting new ideas and methods flow through your head from which you might find a solution. It's actually quite interesting: when your brain (or specifically an area of the temporal lobe, which is in the side) is ready for ideas, an EEG trace would show waves called 'alpha waves' that have a long wavelength, but has soon as the idea comes that same area switches into 'gamma wave' mode so it can focus on the idea. That same sort of pattern is identified in creative processes like playing music.
However, zzdfa brings up a decent point (although appianway is not asian I think so won't appreciate the stereotype

) and it's also true that a lot of smart people don't play music and a lot of top musicians aren't particularly strong academically, but yes there appears to be a correlation