From Wikipedia, Hunkerin’ further explained:

Considered by authorities as much preferable to the craze of the previous year, phonebooth stuffing, people hunkered for hours at a time on car roofs, in phone booths and wherever people gathered.Life referred to it as “sociable squatting.” Different styles of hunkerin’ were reported as “sophisticates” tended to hunker flatfooted while other hunkered with their elbows inside the knees.

Reasons for the popularity of hunkerin’ included the ability for large groups of people to participate together peacefully to discuss issues such as politics or sport. When asked about the popularity of hunkerin’, one hunkerer said hunkerin’ was

“A respite from a world of turmoil. The main purpose of hunkerin’ is to get down and hunker together. It’s a friendship thing: get your friends to hunker with you. The man you don’t know is the man you haven’t hunkered with.”