When children are too young to train pokemon, their mothers typically give them two pieces of advice: "Don't go into the tall grass" and "under no circumstances make a Spearow mad." It would do more trainers well to heed the second piece of advice, even once they get their first pokemon; alas, many are too hotheaded to listen.
Spearow are a rare and cowardly pokemon, but quick to anger; if a trainer is too weak to capture or kill one they antagonize (and they are quick to anger) a Spearow will call for backup from its friends, and young children who struggle with one of them now battle a whole flock.
Sometimes, such as in the case of Red, trainers have already captured an electric, ice, or rock pokemon and can use its type advantage to survive, albeit typically in critical condition and dire need of a pokemon center. Far more often, young trainers are pecked and scratched to death, and their bodies along with those of their pokemon feed the victorious flock for an entire week.