After a week of unusually heavy rain, we had two 60 foot pine trees that were already leaning about 20-30 degrees from verticle finally fall one windy day. The root structure ripped right out of the ground. My dogs used to take a nap under those trees because of the shade and the nice breeze. Now, they won't go anywhere near there. It appears that good root structure and solid ground is the key to keeping a leaning tree from falling over. At least mark off a safety zone. We were lucky. The trees fell in an area that damaged nothing. If not for the wind that day they might have taken down a fence and my wife's rose garden.