Sounds like a simple matter of seal failure. I am sure others would give the same advice: check your mason jars every so often. Make sure the lid is still tightly depressed down in the center (when you push down on it, there should be no springiness at all) and give the edge of the jar a sniff (if you can smell tobacco, it ain't sealed). Also, use fresh lids for each jar if at all possible so as to avoid potential problems down the road (I reuse lids occasionally, depending on how long I anticipate storing the tobacco, but extra lids and bands are so cheap that I often just toss them after a single use).