Spoiler alert!!! End of movie question....
To me it seemed as though the father intentionally killed himself. He was so grief stricken that no one would blame him. Yet, the other side of the coin is the fact that he was such a salt of the earth man and such a hard worker that it wouldn't make sense. Evidence says maybe he did kill himself though. The insurance papers the son saw after the suicide were fairly convincing to me Tully going back to the truck and realizing his dad could have triggered the truck bed to land on him and kill him....what do you think?