Nodules on tree

This looks like burls on the tree trunk. The cause of most burls cannot be explained. This can be a physical response to insects, bacteria, fungi, mistletoes, some genetics, or environmental injury, such as freeze damage. Some trees that may have burls include oaks, maples, cherries, hackberries, etc.
Here is more information on our website.
http://extension.umd.edu/hgic/burls-trees-and-shrubs

Marian