Unequipped monsters are Special Summoned to the side of the field of the player whose effect caused the monster to be unequipped.
All Union Monsters and some other cards with similar effects can unequip themselves from an equipped monster if they have not used their own effect to equip themselves to a monster in the same turn. "Formation Union" and "Combination Attack" can both unequip Union Monsters.
When a monster with an effect to unequip itself is equipped to a monster, it can unequip itself even if it was not equipped by its own effect, unless specifically stated otherwise (e.g. "Union Rider").