Why did the radiator overflow bottle overflow?
The overflow bottle may overflow if: 1. The car is overheating -
this means after the coolant in the radiator gets heated up and
expands it flows to the reserve bottle but the radiator doesn't
cool down to suck back the coolant from the overflow bottle, but
just keeps on sending coolant to the bottle. 2. You may have put
excess coolant in the bottle - hence heated coolant from the
radiator didn't get enough space.