Why does solar lead to increases in grid voltage?

Why does solar lead to increases in grid voltage?

When your solar system is producing more power than your home is using, it sends the excess back to the grid. In order for power to flow from your home to the grid, the voltage from the solar inverter has to produce a voltage that is a couple of volts higher than the grid voltage. 
There is some discussion that the grid voltage supplied by distributors (the DNSP) is already too high, resulting in inverter shutdowns. Ageing grid infrastructure is not helping the situation.