Ever wonder just how much strength you are fighting with outdoor materials?
It is going to depend on your sail area and angle. A 5kt wind will give a 9x10 canvas 10 pounds of lateral force. double the wind to 10kts and you get 38 pounds of force. 15kts and you get a wopping 87lbs of force.
Basically to keep it from tipping you need an equivelent amount of weight pulling against it to maintain position. So either tiedowns in the opposite direction of the wind, or double the loading on each stand.
Here is a handy website to calculate sail loading, and your canvas is basically a sail.