I've got several trees on my land with similar growths and have wondered the same thing. I suspect some kind of virus causing abnormal growth, but that's only a guess. I'll have to get a picture of one particularly impressive burl at the side of my house.
I am guessing it is Burl in the making...Here in Utah Burl is on almost every tree it seems like. Especially the Box Elder trees. And the Burls are Massive. Gonna get me a chainsaw (Stihl MS 261) and see if I can help some people get those ugly trees out of their yard~!
They are som common, people use them as mail box post's....So Sad