Probably just a vigorous fermentation. If you have minimal amounts of headspace in your fermenter, the krausen can definitely get into the airlock, and potentially clog it. Because of this, I always use a blowoff tube for the first couple of days of fermentation. I had a lid blow off an ale pail before, and fly clean across the room. I was cleaning trub off the ceiling! I've also heard about glass carboys exploding, so I like to be safe.
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