From looking at the nhk video, a bomb or fuel tank explosion (as the root cause) is highly unlikely.
Bomb: bombs are placed in the passenger cabin or cargo, for obvious reasons. If the timing's off, they are still powerful enough to cause a significant disruption in the cabin. Every indication so far is that whatever set this off was from outside the cabin.
Center Fuel Tank explosion: same deal. People would not be evacuating from an intact aircraft. Now, it is likely that the center fuel tank "exploded" at some point. Take an "empty" (=fuel-air mixture) fuel tank, start a fuel fire underneath it, and see what happens.
For that matter, there are all kinds of interesting things that happen in fuel fires, as the vapor is considerably more flammable than the liquid. "explosions" and "fireballs" can occur from an action as simple as increasing the amount of fluid poured on the fire (such as a second fuel leak).
Also, the aircraft appears to have been evacuated on the right side only (with the exception of the Captain being "assisted" out the left window), without the overwing exit. Thus, the evacuation implies a left-side, external incident.