The explanation from redditor funktapus:

Coca-Cola is filled with dissolved CO2. The CO2 can escape (creating gas) if the partial pressure of CO2 is too low next the liquid. That's why Coke goes flat if you leave it exposed to air.

Next we have butane, which is a nonpolar liquid at low temperatures, but boils at about 0 Celsius. When you add a bunch of butane to the coke bottle, it probably starts to form a liquid on top of the coke (it won't mix), which causes water ice to form, which acts as an insulator.

Now, you flip the bottle. Not only does the liquid butane come into contact with warm coke, which causes it to boil, creating butane gas... but the newly gaseous butane doesn't contain any CO2. That causes CO2 to rush into the new butane bubbles and expand even more.

The end result: a bunch of new butane gas and CO2 gas, also known as an explosion.

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