What are the common causes of head shake / tank slapper?

www.facebook.com/home.php#!/davemosstuning Marcelo Delgardo wants to know why his bike got into a tank slapper that put them both on the ground. There are many causes!

• #people_blogs
• #suspension
• #sag
• #geometry
• #tank_slapper
• #head_shake
• #shifting
• #braking
• #deflection
• #crashing