Some of them come from Daily Kos, some from DU, some from anti-FReeper sites that shall remain unnamed.
And some just wander in out of boredom in search of abuse because they are masochistic in nature.
They enjoy being abused, it validates their existence somehow.
it is the Culture of Victimhood the grass-roots leftists are addicted to - they can't feel good about themselves unless they get a routine and regular beat-down at least once daily, so that they can consider themselves "oppressed" and, thus, "good"