What are the possible causes of coulrophobia? Most sources on the internet say that coulrophobia is caused due to the unfamiliar face which often triggers fear to people (who suffer from it). What can be some other causes?
Coulrophobia is a fear of clowns. 

It is a phobia not (yet?) acknowledged by the WHO or the APA as a disorder. Nonetheless it seems to be an accepted phobia in hospitalized children (Meiri et al, 2017).

The Guardian has a very interesting popular scientific account on the causes of coulrophobia. The author brings up the following two convincing arguments why clowns can be scary (and others arguments as well - I encourage you to read the article itself):


The “uncanny valley” effect: A phenomenon whereby things that look human, but are in fact not quite human appear as highly unsettling. Think of a dead body for example. In case of a clown their elaborate make up and weird behavior contradicts our sense of 'normal'. 
Unpredictability is something which causes distrust and apprehension in humans. Clowns are by their very nature unpredictable, as they always act weird and do not adhere to social standards.


Reference
- Meiri et al., Eur J Pediatr (2017); 176(2):269-72