During the summer months Bora Bora gets a lot of rain which is when most people avoid going to Bora Bora because of the wet rainy season.
The rain in Bora Bora and Tahiti also tends to fall during the afternoon and night.
The best time for visiting Bora Bora is November through April which is a good season for Bora Bora and there are less tourists.
During May through October rain is isolated but tourists activity goes up during those times so it can be crowded.