When Tara judged the time was right, she engineered a quarrel and left Willow, hoping that it would push her over the edge. Unfortunately, Willow proved too strong-willed, and tried to give up magic instead.

Remember when Tara's clothes shaped into her body and they were offering Willow some comfort? There were in fact three belts there, YES foreshadowing to Twilight's costume, because Tara is actually Twangel taking revenge on Willow.