The muscles of the body are going to be stiff if you strain too much when playing sports.
In the case of vocalization as well, vocal cord muscles will also become stiff and contract by excessive tension. This will result in unintentionally producing a high-pitched voice when singing or speaking.
There are 2 main factors when making a high-pitched voice, the first is to contract the vocal cords muscles and the second is to extend and thin out the vocal cords muscles. Extending the vocal cord muscles can produce a variety of different sounds, not just one.
In the case of stiff vocal cord muscles from excessive tension, the voice will unintentionally sound quieter (not at normal volume) and also have a shrilling high-pitched sound effect added to it. This is because the vocal fold isn’t vibrating well.
On the other hand, in the case of extending the vocal cords muscles when not stiff, the voice will sound rich and beautiful, because the vocal fold is vibrating well.
The vocal cord is extended by the cricothyroid muscle, this action makes the vocal cord thin. For example picture a rubber band being pulled back. When it is pulled back it also extends and becomes thin. When the vocal cord becomes thin due to normal tension (not excessive), it can vibrate well and produce a good high-pitched voice.
If your voice unintentionally sounds higher-pitched than usual in singing or conversation, then the musculus extrinsic laryngis has been strained by excessive tension.
The above photo shows the vocal cord closing tightly due to excessive tension when vocalizing.