The dominant hand changes when the weapon is swung. With a single handed weapon the weapon is held with the dominant hand (for example the right hand) this switches with a two handed weapon (or tool as this is the same for cutting wood with an axe or digging holes with a pick). The left hand would now hold the weapon and the right would be used guide the weapon and balance the momentum, often by sliding up and down the shaft, the longer shaft the further the hand moves.

In short a one handed weapon can be automatic (you attach it to the hand). Two handed weapons require the hand changes to be animated and the longer the weapon the more drastic the changes need to be.