Haber de

Question

I found the sentence, ¿por qué habría de asustar un sombrero?— me respondieron. I tried looking it up and apparently haber de can be used conditionally to express confusion of a topic. i think this is important as well as haber que, i saw it in a book although i can't remember the sentence haber que is apparently also another form of obligation like tener que

Accepted Answer

Hola David
That's also quite a specific use of haber de. It's used as a question ¿Por qué habría de...? when a doubt about something has risen in a conversation and a person asks this question to clarify the doubt. 
I've added some example sentences to illustrate this use in the lesson.
The other structure that you mention for obligation: haber que... is more basic and is in this other lesson. 
I hope it helps.
Saludos