106.11 To "tap [a permanent] for mana" is to activate a mana ability of that permanent that includes the symbol in its activation cost. See rule 605, "Mana Abilities."

An ability that triggers "whenever you tap a creature for mana" triggers only if you activate a mana ability of a creature including in its cost. Mana abilities that don't include the T symbol and instead say "Tap an untapped creature you control" or similar won't cause Leyline of Abundance's second ability to trigger. Similarly, it won't trigger if you tap a creature to activate a mana ability of another object (even if that mana ability also includes ).