Over-carbonation and gushing
Updated
The customer opens the bottle and loses half of it, which reads as a defective product whether or not the carbonation is wrong.
Two different complaints
Gushing: the drink foams violently on opening at a normal carbonation level.
Over-carbonation: there is genuinely too much gas in it.
They feel identical to the customer and have different causes and different fixes.
Gushing
Caused by nucleation sites: something in the liquid for bubbles to form on.
Particulates, pulp, protein haze, and in a live product the culture itself.
Also by agitation, which is why a bottle that traveled badly gushes and the same bottle rested does not, and by temperature, since a warm bottle holds less gas in solution.
The fixes are clarity, a rest before sale, and a label instruction to chill and not shake, which is a real instruction rather than a disclaimer.
Genuinely over-carbonated
Either too much gas was put in, or the bottle made its own.
The second is refermentation and is the more serious, because gas is the visible half of a problem whose other half is alcohol and pressure.
So a batch that is gassier than specification should be investigated as a fermentation question rather than adjusted as a carbonation one.
Fixing it
Measure carbonation rather than judging it, at start, middle and end of a run.
Set the specification against the warmest temperature the product will see rather than the filling temperature.
Reduce nucleation: filter, or let the product settle, or accept a cloudy product and say so.
And put "chill before opening" on the label, which is honest, useful and the cheapest fix available.