It evolved from exposure to a special magnetic field. Three units generate magnetism.
A group tried to use scientific means to make MAGNEZONE evolve, but their efforts ended in failure.
Exposure to a special magnetic field changed MAGNETON’s molecular structure, turning it into MAGNEZONE.
A group tried to use scientific means to make Magnezone evolve, but their efforts ended in failure.
Sometimes the magnetism emitted by Magnezone is too strong, making them attract each other so they cannot move.
— Pokédex entries (via PokeAPI)