Certified GKH Gardening Expert
Trees generally sucker when they are under stress. Are the branches still alive? It could be your tree's top growth has dies but the roots are still alive, in which case it continues to grow through it's suckers. This article will help determine if the tree's upper growth is still alive: https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm
You can remove the dying growth and allow the strongest sucker to grow, if desired.