Complex sentence: It is assumed that when those that are working on trees are killed by the tree these deaths are almost always the result of the work that was being done and not because the tree or a part of the tree failed on its own accord.
Split into simpler sentences: It is assumed that when those that are working on trees are killed by the tree. These deaths are almost always the result of the work that was being done and not because the tree or a part of the tree failed on its own accord.