My theory (as an ex-psychologist) - Research has shown that we all have various combinations of the different human aptitudes. No one can do everything, and it is hard to know exactly what all the aptitudes we have are. Some are apparent, but others can be hidden unless you try everything there is to do in the world.
In the case of musicians changing - they obviously have some of the major music skills, but perhaps not the right ones for what they were doing, or the instrument they were playing was not satisfying.
For violin, for example, you need extreme finger dexterity and very good pitch discrimination. For sax, a particular type of mouth is advantageous - same for horns. For drums, you must have rhythm. If you lack the natural ability, playing them will be difficult and unfulfilling. We have all seen the tryouts on American Idol where the people truly have no idea how bad they are. Most really cannot hear it!
I started as a guitar player, but I found I had limited finger dexterity but strong rhythm, so I changed to bass. Ideally, I should have become a drummer as I will never be able to play "fancy" bass no matter how hard I practice.
When I was a teenager, my mother found a research foundation where you can have your natural abilities tested and measured, and direct you in the right direction for your life. Of course I didn't listen to them because I was 19 and knew everything, but later in life I found out that they were 100% correct. I am very very very lucky that I just happened into doing the right thing to fit my abilities (audio engineer) or I would just be another one of the masses that just works for money and hates their job instead of enjoying every day as I do, and doing it well, besides.
Check this out -
www.jocrf.org I have sent more than 100 people over the years, and they have all thanked me. It can explain a lot.