I believe the most common cause of dessert on earth is humans. The process is actually called desertification. It usually involved deforestation, poor farming practices, poisoning the ground, and a natural tendancy to low precipitation.

However you state that this is a land that is "one" with nature. 

#Option #1. Past
Perhaps the people are one with nature because of what they did to this island.

#Option #2 Cataclysmic force. 
An earthquake, **meteor**, tsunami, volcanic eruption, or magical beast could have caused this.

#Option #3 End of the world.
I think the sun is actually supposed to get colder as it gets older but a decaying orbit might draw the planet closer to the sun. This would result in storms, heat waves, unpredictable weather and general chaos. Wouldn't likely spare the other islands though.