+ they "hacked" labours database in the first place. There is a saying about glass houses and stones that could be considered apt.
I doubt this was hacking, more likely it was leaked by a pissed off associate of Slaters.
Hacking is actually really hard, if it was hacking it would be either via a dictionary attack or perhaps there was an element of social engineering, likewise labours database wasn't hacking it was just guessing a URL for an unprotected file.
Moral of the story is use lastpass.com