The bolt of my pedal's crank arm broke, what could be the causes?
The "bolt" that broke is not a part of the crank. Your bike has a so-called square-taper bottom bracket and one side of the axle broke off. This axle is a solid metal part that runs from one crank arm through the bottom bracket shell to the other crank arm. It cannot be serviced and should not break. The most likely reason is metal fatigue.

You'll have to buy a new bottom bracket if it is a cartridge type or at least a new axle if is the cup-and-cone type.