metals hv low ionisation energy, hence they lose electron easily. similarly, size of metals r comparatively large, so the bond strength of metal-oxide bond is low. der4 by applying very small amount of energy d bond cn be broken. after dis d metal atom is reduced by taking electron.

  correct me i m wrong n rate if useful........

yes, cations r generally lewis bases which accept electron pair i.e. they get reduced. But in case of metals having only 1 electron in its valence shell like Na₂O, Na cant accept electron pair, they accept single electron.