Leather contains moisturising and lubricating oils which were added in the tanning process. These oils are gradually lost as the leather is used. The once plump and flexible fibres of leather gradually become thinner and more rigid.
Instead of flexing and stretching, the fibres become tight and stiff. In a similar fashion to bending a piece of metal repeatedly, they will eventually weaken and break, causing cracks.
Using an oil such as Vanner & Prest® Neatsfoot Compound will revitalise the fibres in the leather so that they can move freely again, much like oiling an engine.
As a guide, apply one or two thin layers to the absorbent flesh side of the leather. The leather will become strong again, less brittle and less likely to snap.