For many males working in blue collar, labor-intensive positions, their power, strength, stamina and (often) tenuous financial position is the only evidence of their masculine identity.
They are angered that this has been _taken away_ by their injury. They are angry with their doctors, angry with their nurse case manager, angry with their adjustor_angry with their family. Often, the anger is due to their belief that they are now seen as less masculine, as weak, as dependent, and they feel their masculinity has been impugned.
They are, in fact, now quite dependent upon others, and this often enrages them. But they can do little with their rage since to express the anger would threaten their needs being met. So they sit on the anger and are often misidentified as being depressed because the symptoms (irritability, sleeplessness, overeating, etc) are similar.
If the underlying problem were impotent rage, treating it as though it were depression would not be effective.