The flame is too high
Updated
The most direct safety failure in the craft, and one of the properties ASTM F2417 addresses.
What too high means
A flame noticeably taller than the small, steady teardrop a well-wicked candle produces, in a room with no draft.
Often accompanied by flickering, a mushroomed wick tip and soot on the glass. Those three arrive together because they have the same cause.
The causes
A wick too large for the wax and the vessel.
A fragrance load above what the wax holds, delivering extra fuel to the flame.
A mushroomed tip, which enlarges the effective wick and makes the problem compound.
An untrimmed wick, which is the customer-side version of the same thing.
And a draft, which is the only cause that is not yours.
Why it matters more than it looks
More heat into the vessel, which is how glass fails.
More heat above the candle, which is what reaches a shelf, a curtain or a cabinet.
A larger ignition source if anything falls in or blows near it, which is the secondary ignition question.
It is also the failure a customer photographs, and it is the photograph that ends up in a review.
Fixing it
Drop a wick size and retest all the way down.
If dropping introduces tunneling, the load is too high or the series is wrong, and the answer is less fragrance or a different wick construction.
Put "trim the wick to 6 mm before each burn" on the label, because it is genuinely useful. Do not treat it as the fix.