First of all you have to check the root which has started circling. Then notice that there is a dark portion at the bottom of the trunk. You can prevent this problem by avoiding to plant too deeply and not to leave in the pot for too long. You have to plant it at the right depth. Now check the roots of the bigger tree that has died. You shall notice that the roots have started circling around a part and then they stop the circulation to the top of the tree and then the tree dies because it was planted in a small container. You can avoid it by pulling out the plant and planting another one to replace it. Another thing you can do is that you can use a knife to cut off the root part that has started circling and then bury it again to make it grow normally. Sometimes the landscaping equipment heavy machinery may also kill the tree by pushing in the dirt that stops the roots from growing and cutting off the circulation. Also you may think that it is good to dig out a large hole and then plant the tree at the right level thinking that the roots shall grow better. But when you water it the plant sinks in and then it dies. Another problem is that when you have planted a tree and then added a twine or rope to hold it in position. However you forget to remove it in time so the plant gets strangled by it. Another example you forget to remove the wire skates and therefore the tree dies due to it. In one more example you shall notice that the tree was planted in the tin gallon plastic pot and it stopped its growth when it grew bigger. That's it.