A. Mixture of Pressure Hull Materials Failed Titan?

Diagram of Titan's pressure hull/chamber courtesy Washington Post.
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One path (A) of thinking about Titan's implosion (into bits) is that it happened instantly, before it hit the seafloor. This is discussed below. Another path of thinking is Titan imploded following a distortion of hitting the seafloor. Either way this may have resulted in the one loud sound picked up be the US Navy.

It could be that at each end of Titan's pressure hull/chamber, the Titanium hemispheres you can see above, were  generally able to withstand pressure down to 4,000 meters. 

1.  Except for the Titanium front hemisphere's porthole (of glass or plastic?) which was only rated as being safe to "1,300 meters" - quite a physical gamble even before the first of Titan's several dives to around 4,000 meters.

Usual materials for a pressure hull are Steel Alloys and more rarely Titanium which are suitable because they are partly malleable/flexible, so able to slightly contract, under pressure of water, as a sub  descends and slightly expand to their original shape as the sub returns to the surface. 

Submarines using steel in particular, have been tested for over 100 years, with each submarine undergoing hundreds of contraction and expansion cycles. This simply didn't happen to Titan's mix of pressure hull materials. Titan's pressure hull weaknesses may have accumulated over its few previous cycles.

2.  As well as the porthole the major weakness of Titan was the mid-section cylinder of around 130mm of carbon fiber, basically a type of plastic. Unlike steel or Titanium, carbon fiber for submarines/submersibles only has a short track record. Titan's carbon fiber was little tested, therefore little understood. It was inflexible hence tending to crack or tear rather than shrinking slowly (like metals).

3.  The use of two radically different materials (Titanium and Carbon Fiber) in the one pressure hull may, in itself, have led to differing, incompatible, rates of contraction and expansion, leading to ultimate destruction.

4.  Another fault, if the diagram above is accurate, are relatively sharp inward-facing "corners" of the pressure hull where the Titanium hemispheres meet the edges of the carbon fiber cylinder. Corners should be avoided where pressure is concerned (even outward facing corners of the Comet airliners' square windows 60 years ago caused metal fatigue under pressure in another direction, leading to destruction).   

So the inward facing corners of the pressure hull may have caused metal and carbon fatigue and/or a "spike" (also in relation to 3.) of highly pressurized water between those two materials.   

See my subsequent alternate theory of June 29, 2023 "B. Collision With Seafloor Destroyed Titan?"