Recently diagnosed with intersection syndrome, I'm wondering what could have caused my problem?

This is a problem due to overuse. Repeated actions of the forearm, wrist, and thumb causes rubbing where two thumb muscles cross over the top of two wrist extensor tendons. When the area becomes inflamed, you feel pain when you move your wrist because the swollen tendons are rubbing up against the thumb muscles.

Wrist extensor tendons work like the bow used by violin players. The wrist extensor tendons are like the bow, and the thumb muscles are like the strings. As the wrist curls down and in, the wrist tendons rub back and forth against the thumb muscles. The friction builds up, much like the effect of rubbing two sticks together. This leads to irritation and inflammation of the tenosynovium covering the wrist extensor tendons.

The wrist extensor tendons are strained by any activities that cause the wrist to curl down and in, toward the thumb. These wrists movements are especially common in downhill skiers when they plant their ski poles deeply in powder snow. The same movement is involved when pulling a rake against hard ground. Racket sports, weight lifting, canoeing, and rowing can also stress the wrist extensor tendons.