You want beans with that?
I can think of two natural processes that might generate a very large bubble of gas under Europa's ice.
Air under arctic ice
and no, flatulence isn't one of them.
The first would be an ocean completely saturated with gases (you can pick your gases but some of them are not as suited to this as others) encountering conditions (lower pressure / higher temperature) which causes some of those gases to precipitate / out gas.
If the process needs to be abiotic, then the most likely composition of this gas would be the volcanic gases (shown below). But if you have a lively biosphere, then it could be something like $O_2$ or other biological gases.
The bubble would be created when the water conditions suddenly changed. For instance, if the layer of ice above it suddenly thinned and the temperature increased (say due to ice crustal deformation).
The second would be volcanic fissures releasing gases beyond what the ocean can absorb (another case of a saturated liquid). On Earth the volcanic gases most likely to be encountered are:
$CO_2$ Carbon dioxide
$SO_2$ Sulfur dioxide
$N_2$ molecular Nitrogen
$CO$ Carbon Monoxide
$H_2$ molecular Hydrogen
The problem is this condition is not stable. The gas is MUCH lighter than ice and will flow through any cracks to reach the surface. I wouldn't expect it to last long. Since I have no background in this subject I couldn't tell you how long is "not long".
It could fail by gradual (or sudden!) deflation, gradual reabsorption by the water, or most spectacularly by the sudden break-up of overlying ice.
I hope a temporary bubble is good enough for your purposes.
Since the ice above this air has no support, there are mechanical limits to the amount of air in this space. Under great pressures (like a mile or more of overlying ice), ice tends to flow like plastic.
This is another reason I think the bubble would be a temporary feature. The larger the bubble the shorter the time it could exist.