My opinion is that cancer probably was the cause but taking 50mg of prednisone would have suppressed the immune system's ability to fight cancer and could have been a complicating factor. It is not an addictive drug but such a large dose does shut down adrenal production of cortisol (natural prednisone) and most patients are tapered slowly off such a high dosage that had been taken for 7 weeks. I know that some protocols for prednisone give even higher doses for shorter periods of time and taper within a few days. Long term use can be tricky and usually is tapered slowly to give the adrenals a chance to produce cortisol again.
Prednisone is not normally prescribed if there are bleeding issues but your fathers cancer was not diagnosed until after the fact. Again, prednisone might have complicated issues but I would question if it caused the bleeding and his death.
Keep in mind that all of us on this forum are lay persons and our knowledge is concentrated on auto immune diseases and issues surrounding lupus and other related diseases and treatment. If you are really concerned then have an independent pathologist examine the deceased.