Puppies removed too early from their mother and litter - mates may also develop aggressive traits because they have missed out important life lessons such as bite inhibition and other important social behaviors. (puppy-training-solutions.com)
Start socialization early to prevent aggressive traits from forming. (petguide.com)
Bad breeding practices may or may not pass on a genetic tendency towards aggression, but many irresponsible breeders looking to produce «tough» dogs will specifically breed for aggressive traits. (thesprucepets.com)