One very common problem we come across is a tree being buried too deep, or having mulch piled up onto its trunk.
When this occurs it traps moisture up against the trunk, which is very attractive to fungus and other decay agents. Since the circulation of nutrients occurs just beneath the bark, it can be easily disrupted by this decay.
As a general rule, you should be able to see the root flare at the base of a tree. If it looks like a telephone pole sticking out of the ground, it is too deep.