Generally, a state can only tax its non-residents on income from or assets in that state. When you are no longer a resident of a state, and if you will no longer have income from or assets in that former state, it loses its ability to tax you. Thus, to prevent the loss of revenue from your departure, the former state may attempt to treat you as a resident, unless you prove otherwise. Establishing that you are a Florida resident proves that you are not a resident of the former state!