The vessel cracked
Updated
A container that fails while lit releases a pool of burning wax. It is the failure with the highest consequence and it is almost always preventable.
The worst outcome in the craft
Glass can crack, and it can break apart. Either way there is now liquid wax and a flame on a surface that was not chosen for them.
It generally happens late in the burn, when the flame is closest to the base and the glass has been through several heating cycles. That is precisely the part of a burn test people stop watching.
Thermal shock
The main mechanism: a temperature difference across the glass large enough to exceed what it can take.
Hot base, cool wall. A hot candle set on a cold stone surface. A draft cooling one side while the flame heats the other.
Annealing is what determines how much of this the glass tolerates. ASTM F2179 sets annealing, temper and thermal shock requirements specifically for glass containers produced for candle use, which is why whether a jar was made for candles is a safety question rather than a marketing one.
The other causes
Over-wicking. Too much heat for the vessel. A common cause and entirely yours.
Burning past the recommended stop point, where the flame sits almost on the base.
A flaw at the base, a scratch or an inclusion, which becomes the crack origin once stress is applied.
A shape that traps heat: a sharp shoulder, a narrow base, a thick uneven wall.
Preventing it
Use glass specified for candle use, and ask the supplier the question directly.
Wick so the vessel stays comfortable to hold. Keep a hand on it through the test.
Burn the test candle to the very end and see what happens at the base.
And label the stop point: leave about 10 mm of wax, which is on the care card of every well-made container candle for exactly this reason.