Acetylene-filled balloons explode - GIF

[DIYSwede]

Acetylene is heavier than air, its probably methane / natural gas.

Nope - acetylene is 10% lighter ( than air) at 1,095 kg/m3 @ 16 C and 1 Bar.
Density of air at 20 C and 1 Bar, is between 1,1934 @ 0% RH and 1,2039 @ 100% RH

For most practical uses: ρ = 1,25 kg/m³

Ignition was probably caused from static electricity potential differences between the balloons and/ or their respective holders.

2 cents & Cheers
Johan