Decompression causes many things to happen. In less severe cases, maybe only a few windows get blown off and the aircraft can still fly. In more deadly scenarios, maybe the integrity of the fuselage gets compromised. A section of the skin is torn, but its still holding on to the main body. But the force of the air gushing out from within forces the hole to enlarge, plus the a/c flying a 500kts pulls everything apart.
Eg, when the UA 747-200 in 1989 decompressed in mid air, the cargo door pulled along with it the side of the fuselage above it. An Aloha 737-200 had a small section of the forward roof section of the fuselage fail in flight but the small area of skin pulled along with it the entire roof structure of the first half of the aircraft as the area of failure rapidlly expanded!
Thus, probably somewhere on the CAL 747-200 failed. Say in the middle of the a/c, tearing the fuselage apart, one section leading to another. All this while, large sections of the fuselage hits the ver stab, maybe that failed as well. At 500kts, its not surprising even the intact parts of the fuselage not torn by the decompression, start to fail as well... Depending on how the event happened, some parts of the 747-200 might have been weakened and several essential points now take on more load than they were built to withstand (now we know there are 3 of these points), causing the aircraft to break into 4 main pieces in the air.
The Aloha 737's weak point was the belly area immediately forward where the leading edges of the wing meet the fuselage. After the roof and the sides was torn, there was only the belly connecting the stricken part to the rest of the aircraft. In fact, the strickened section was bent down by a few degrees. If the 737 was airborne any longer, the a/c would have broke into 2.
The UA case was different. The area of damage was smaller when compared relatively to the Aloha 737, hence, reacted better to the decompression.
All this can happen within a few seconds to a few minutes. The UA decompression happened so quickly that the Business Class passengers didn't even see their neighbours and the cabin walls get sucked away - it was BOOM and there it was, the gaping hole. The Aloha case, passengers saw a FA get sucked through the hole in the roof before the collapse of the entire structure. Thankfully all their seatbelts were still fastened.
Hence, it does not "just break apart", there is a chain of events leading to this.
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