All you needed to do is do a quick search on MFK or at least read my gulper thread where I dutifully reported this. This phenomenon is known, normal, observed regularly, and reported too. It startled me too the first time mine did that.
IIRC the best explanation I know of is that they fill their stomach with water, perhaps to rinse and/or to realign the organs after a super meal or to push the feed being digested along the bowels when the yawning is not enough... or all of the above.
Then probably puke out the water as it'd not deflate so quickly naturally.