The lenght of a person's life is already fixed at birth. There are three reasons why you may not be able to finish your lifespan:

• You may suffer an untimely death in an accident.

• You may suffer an untimely death in an attack.

• You may not be able to finish your lifespan due to exhaustion of merit - using up your store of the good karma that keeps this lifetime going.

Everyone is restlessly busy, having nothing upon which to rely. Whether moral or corrupt, of high or low rank, rich or poor, noble or base, all are preoccupied with their own work. They entertain venomous thoughts, creating a widespread and dismal atmosphere of malevolence. Subversive activities are planned, contrary to the universal law and the wishes of the people.
Injustice and vice inevitably follow and are allowed to run their course unchecked until evil karma accumulates to the limit. Before they expect their lives to end, people meet sudden death and fall into evil realms, where they will suffer excruciating torments for many lives. They will not be able to escape for many thousands of kotis of kalpas. How indescribably painful! How pitiable that is!

Shakyamuni Buddha

If you are a petty person, people will hate you, and disasters will come to you. Your good fortune and lifespan will be cut short and you will suffer many losses.

Lung-shu

Every evil deed use up your store of good karma. If the store of good karma is used up, you will meet sudden death.

You were endowed with a human body to think and act like a divine being. If you fail to make spiritual progress in this life, you live a life of meaninglessness; in this case your good store of karma could be used up anytime, and you will meet an early death.

that use up the store of good karma:

The Five Precepts
1. No Killing, also no eating of animals
2. No Stealing
3. No Sexual Misconduct
4. No Lying
5. No Taking of Intoxicants, also no heavily consuming of alcohol

Ten Evil Acts
01. Killing
02. Stealing
03. Indulging in sexual misconduct
04. Lying
05. Committing slander
06. Using coarse language
07. Indulging in empty chatter
08. Harboring covetousness
09. Using angry speech
10. Holding wrong views

The Five Grave Offenses
This offenses cause rebirth in the Uninterrupted Hell. Those who fall into it suffer, die and are instantly reborn to suffering without interruption.

1. Killing one's father
2. Killing one's mother
3. Killing an Arhat
4. Causing dissension within the Sangha
5. Causing the Buddhas to bleed

Arhat: a buddhist saint
Sangha: the buddhist community