The bolt of my pedal's crank arm broke, what could be the causes? It broke while as I started pedaling when a redlight turned green. 

In the pics it is possible to see that basically the metal broke! Was it just a poorly constructed product or are there some common causes that explain why this happened (like i should have unmounted it and cleaned the dirt out of the screwing lines sometimes or else..). 

I wouldn't want this to happen to my other pedal.






Original pictures: https://m.imgur.com/a/awW3NgK

Thx
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.