Large Tree Growth

This tree developed a mutation that caused a growth called a "burl." A burl looks like a large, round growth with a rough texture. Often burls are underground, but this burl is prominently visible from the tree trunk. 

An burl forms when the tree’s growth hormones get disrupted and the metabolism of the tree is disrupted by another organism, such as a virus, fungus, or bacterium. Some of this bacterium can even carry extra DNA to infect the tree. 

Other sources below indicate it can form when a tree gets damaged and callus tissue develops, folding over on itself. Either way this can be a genetic phenomenon that alters how the tree grows. It is not all bad though. Because plant cells have cell walls, this mutation does not spread - only grows. In addition this is genetic trait/alteration has helped trees survive from less-ideal environmental factors, because If the tree then dies, a new tree can sprout from the burl. 

As a result of the unique shape of the burl, wood from these trees can be highly sought after for woodworking.