The story
Uncertain. One popular theory is that competition mainly drove the term; publishers "fought like bulldogs" to "get out editions that would catch the mails going out of town." Another theory suggests that it comes from the nautical term dogwatch (“evening shift”), as printers had to work late in the evening to put out an early edition for the morning paper. Numerous other theories have been proposed.
Etymology from Wiktionary, CC BY-SA 4.0.