One theory is that poor blood circulation is the cause. In other words, stiffness is caused by poor circulation of blood in the shoulder area. The other is that it is caused by muscle tension in the shoulders.
Some believe that both of these theories are correct, while others believe that stiffness is caused by a strong pull on the muscles of the shoulders for some reason, and the definitive cause is still unknown.
If you say that your stiffness is caused by prolonged use of a word processor or computer, or if you say that your stiffness increases with age, it is probably due to poor blood circulation, while if you say that your stiffness is caused by being stiff in front of an important person, it is probably due to muscle tension.
Please consider these causes in your own way. The real cause is still unknown.