I'm not positive, but I might imagine that it's that the solar power generated by that particular grid either equals or exceeds the amount used by it. If there's multiple grids it could be that the residential area is nearly self sustaining during the day, and they don't have the capability to move that generation successfully to the other places that need it because the grid interchanges weren't built with that in mind.

That's exactly what it is. See presentation here: http://www.heco.com/vcmcontent/StaticFiles/pdf/LVM/LVMMECOPu...

It appears that there are neighborhoods in Hawaii where the PV generation capacity is approaching the max load of the circuit.