Carcinogenicity of polynuclear hydrocarbons and benzene rings
1.Why are they carcinogenic:

From my point of view as a medicinal chemist, polycyclic aromatic hydrocarbons compounds tend to be metabolized by specific cytochrome P450 (CYP 450) isofroms to form highly electrophilic species, which are highly reactive and can react with the DNA of the cell and damage it. Specifically, they can react with the nucleophilic nitrogens in the DNA bases such as adenine.
To see more detailed mechanism of the formation of the electrophilic species from the compounds, check this article: https://socratic.org/questions/how-do-polycyclic-aromatic-hydrocarbons-cause-cancer

2.Is there a reason for a given organic compound to be carcinogenic?

Mechanism for carcinogenicity can vary, in general, any compound damaging or altering the DNA can be carcinogenic. For example, highly reactive electrophiles (such as formaldehyde).