See  A Review on Structural Failure of Composite Pressure Hulls in Deep Sea.

Quoting,

With the increase of the diving depth of the submersibles and the thickness of the composite pressure hull, the structural failure of the pressure hull may change from the overall buckling to gradual material failure. As a result, the design concept of the structure has to be changed. In addition, the large thickness composite pressure hull used in the very deep sea also has its unique failure mode, snap buckling. This problem appeared in the application of deep sea pressure pipelines in the 1980s, and has received more attention and research since then.

For specifics of the Titan design, see Composite submersibles: Under pressure in deep, deep waters. The thickness of the hull was 5 inches.

A little bit of intuition.

• The metal of a conventional hull has strength in all directions.

• A carbon fiber / epoxy hull has high strength within the fiber layers, but not between them.

• The thicker the hull, the more stress between the layers.

Hence, SNAP!