Hypothesis
Well since hydrogen is a gas thats 4 times lighter than air Its molecules are very spread out and bouncy and when Hydrogen came into contact with heat it either evaporated the molecules or the spread out even farther cause the flame to trail with it making the cloud of flame.
Well since hydrogen is a gas thats 4 times lighter than air Its molecules are very spread out and bouncy and when Hydrogen came into contact with heat it either evaporated the molecules or the spread out even farther cause the flame to trail with it making the cloud of flame.
Hypothesis (Update)
When filling the balloon with hydrogen gas the molecules were very light and spread out and when the flame came in contact wIth the balloons the oxygen atoms from the fire started combining with the hydrogen atoms unstably. This means the atoms that were spread out in the balloon start racing to reach other oxygen atoms and clashing against each other creating a combustable force rupturing the ballon and making a cloud of combustion.