It fell over
Updated
Less obvious than fire and just as consequential: a lit candle that tips is an ignition source moving.
Stability is in the standard
ASTM F2417 addresses stability, alongside flame height, secondary ignition and end of useful life.
It is the property that connects most directly to the shape of what you chose to sell, which makes it a container decision rather than a formulation one.
Container candles
Usually the stable case: a wide base, a low center of gravity and glass that is heavy at the bottom.
The exceptions are tall narrow vessels, which look elegant on a shelf and tip on a table, and anything with a rounded or footed base.
Test it as it will be used: on a flat surface, brushed with a hand at the height a sleeve would pass.
Pillars and tapers
Here stability is entirely about the holder, which you often do not supply.
A taper that fits loosely in a holder leans as it softens. A pillar burning unevenly develops a thin wall on one side and can slump.
Both are the reason free-standing candles carry more instruction than container candles do, and the reason most small makers stay in containers.
Ensembles
If you sell the candle with a holder, a plate or a lid used as a coaster, you are selling an ensemble and the standard treats it as one.
Test the combination rather than the candle. A stable candle in an unstable holder is an unstable product, and the customer will not distinguish.