It's purportedly called a "crawl" because the participants are literally crawling from pub to pub after getting drunk at the first few pubs. According to the Oxford English Dictionary, the term (including variations like 'gin crawl' and 'beer crawl') has been in use since the late 19th century. A common reason to hold a pub crawl is celebrate the birthday of a person who reached the legal drinking age. The pubs chosen for the route can be chosen according to a theme.