Hi Mark, I'm guessing the rise you see when you open the jar is related to the release of fermentation gases that have accumulated in the jar. These gases are likely causing pressure on the level of the starter, and their release allows the starter to rise upward. I wouldn't worry about any of this, except you may want to consider housing your discard in a larger container, so there is plenty of room for both the starter and fermentation gases. Opening the jar now and then to release the gases should also be helpful. I would also recommend throwing out your discard every few weeks and starting fresh. Discard starter doesn't generally spoil, but if it's developing dark liquid on top and has a funky aroma, it's probably not going to add anything particularly positive to your baking. 

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