at the beginning of the chapter, we found out that Mr.Raymond sips only coca-cola from a paper sack, deliberately pretending to be drunk. why does he do this?

Dill takes a sip and discovers Mr. Raymond is hiding a bottle of Coca-Cola in his infamous paper bag. Scout asks why he does such a thing, and Mr. Raymond explains he feels he has to give the population some reason for his odd behavior (being friendly toward black people). Mr. Raymond believes it's easier for people to handle strangeness when they have a reason to explain it. Thus, he pretends to be a drunkard.