Hi Lynn,   There are several different factors that can cause big cankers/wounds in the tree from environmental to diseases (fungus or bacterial. Unfortunately it is not easy to determine without knowing more information and it may require testing a tissue sample if there is a transitional branch that is still alive and starting to die back with a canker associated.  You are welcome to give me a call so we can discuss.  I am headed out of town, but will be back next week.  Thanks