Why did my fondant tear?
Without more details from you...


Try kneading the fondant more to make it more pliable and soft.
Work quickly after kneading.
Roll the fondant a touch thicker.
Try using less icing sugar/cornstarch if you are using these during rolling out.
Freeze your cake for a bit to stiffen the structure to avoid edges cutting into fondant sheet.
Gently adhere the fondant on the cake, holding the side/base of the cake before smoothing the entire fondant sheet onto the cake.