Over-carbonation is more dissolved CO2 than the packaging can hold safely — a real safety issue in bottle-conditioned beer specifically, where a sealed glass bottle has no pressure-relief mechanism if fermentation produces more gas than intended, and most standard beer bottles are rated safely only to roughly 3.5-4 volumes of CO2 before real failure risk sets in.
Where the extra fermentable sugar usually comes from
The two most common causes are a miscalculated or overestimated priming sugar dose, and residual unfermented sugar the brewer didn't account for — an incompletely fermented beer bottled before FG genuinely stabilized, or a fruited or sweetened batch where the added sugar wasn't fully accounted for in the priming calculation. Confirming FG is stable across several consecutive days before bottling, and calculating priming sugar from the beer's actual measured volume and temperature rather than a rough guess, both address the problem at its real source.
A style with a naturally high target needs bottles rated for it
Belgian Golden Strong Ale and similarly high-carbonation styles push toward or past what a standard beer bottle safely holds, which is exactly why genuine champagne-style bottles, purpose-built for higher pressure, are standard practice for packaging that style at home rather than a safety detail worth skipping.