Simply being the owner of the tree does not make you liable.
Generally, No one is liable for acts of nature. So if the tree falls due to a natural occurrence you are not liable. If however you were in the process of cutting down the tree or trimming it when some part of it fell on someone, then you could be liable due to your intervening act.
The above being said, Anybody can sue anybody for almost any reason. Wining the suit however is another matter. They would need to prove that the tree falling on them was your fault.
Anyone can sue you, for any reason. They may not win, but they can sue not only you but anyone else they want to sue.