For current to flow out of your house the voltage inside the house has to be slightly higher than outside. Not by much, but a little. So the inverter has a higher output voltage than line voltage by design. If everyone does this and some of the power has nowhere to go, then the average voltage goes up measurably.

This wouldn't be a problem if the grid had been designed to be able to bring power out of residential areas, but my casual understanding is that this doesn't work very well with existing infrastructure, so with a bunch of extra power that has a hard time getting out the voltage keeps climbing until some inverters hit their safety shutoff.