This has to do with lack of demand during high PV production. This excess energy still has to go somewhere, it doesn't just disappear. This costs the grid operators money, so either they charge you or don't let you feed the excess power to the grid.
If this were the case, why would solar panel owners not stop "selling" power to the grid at their expense when that happens? What incentive would ever exist to pay to put power on the grid?
Note that electricity generated will first be used by the consumer and only excess is sold to the grid.