Rod breakage happens in a few ways.

Genuine manufacturing faults. They do happen and normally will happen very quickly after purchase.Poor qc or a design flaw .

Damage due to car doors sinker hitting it getting dropped etc etc . They are made of a few layers of carbon fibre cloth and resin . Enough of a impact will damage the blank leading to its failure at some stage .

Wear and tear. Most things will breakdown after they have been bent back and forth a few hundred thousand times esp when rods are getting lighter and lighter .Also he joints will wear as well over time.

Overloading . Rods only bend so far before the blank goes from round to oval and the resin/fibres give up. Way too heavy a load or using line way way above the ratings .

Overpowering . Related to the above where you are in the weight range but are simply putting more power then the rod can cope with .

Poor technique. More then a few people break the tip section when they try to pendulum cast and hit the rod way before the sinker is in the right spot to do so.