It's because two reactions take place which create CH4.

The System is feed with C2H6 and H2

C2H6 will be cracked in two CH3° radicals.

This react with hydrogen according
CH3° + H2 => CH4 + H°
1. Methane source and a hydrogen radical
And H° react again with ethane according
C2H6 + H° => CH4 + CH3°
2. Methane source and again a methyl radical

Then the game starts again until all Ethane and hydrogen is consumed or the Termination reaction takes place.

Many thanks for your response! I will look into what you are describing. My tank was held vertically by two clamps, maybe the glass broke from the tension of the clamps and the fact that it happened after I turned the voltage off is just a coincidence?

The failure after the battery being turned off could just be a matter of kinetics. Under Borek's theory of corrosion, corrosive substances generated during the cell operation could take some time to cause enough damage to reach the point of failure.

First is CH4 = k2*CH3°*H2/H° and CH4 = k3*C2H6*H°/CH3° ?

Only this two equations have to be combined.
Substitute H° in first equation with second equation and solve for CH4.

Initiation and Termination is in my opinion not necessary. It's not part of the chain reaction.
Termination have more possibilities like 2 H° => H2 or CH3° + H° => CH4

Thank you again for the response, it seems very weird that this happened especially after turning the voltage off.