Second question, completely different. I had a gallon bottle of grape juice that was almost empty. When I opened it, there was a release of a foul-smelling gas that I think must have been some kind of sulfur, maybe broken down from the sulfite preservative in the grape juice. (Potassium Metabisulfite, says the ingredients label.) Do you know how that could have happened? I've sometimes poured the juice into a crystal glass, not drunk all of it, and poured back the remainder into the bottle.

The metasulfite ion, also known as the disulfite ion, is S2O52− .  In aqueous solutions, under acidic conditions, it converts into the sulfite ion and sulfur dioxide. 

S2O52− ⇋ SO32− + SO2

This is what makes metabisulfite salts useful as preservatives.  Sulfur dioxide gas is soluble in water, and at low concentrations, it can serve as an antioxidant and an antimicrobial agent. 

The potassium and sodium salts of metabisulfite appear to be the most commonly used, and both are important chemicals in brewing and winemaking.  Normally, we think of fermentation as something you want to happen when you’re making alcoholic beverages.  You need microbes like yeast to metabolize sugars into ethanol.  However, you always run the risk of having wild microbes in the mix, which don’t ferment as well as others, and wind up producing an undesired amount of hydrogen sulfide waste.   To cope with this, winemakers and brewers use metabisulfite salts, which introduce a small concentration of sulfur dioxide, which neutralizes any hydrogen sulfide present in the product.

SO2 + 2 H2S → 3 S + 2 H2O

This may seem counterproductive, since you’re just using one smelly sulfur gas to counter another smelly sulfur gas, and you end up with elemental sulfur, which stinks too.  In fact, elemental sulfur is odorless.  The rotten egg smell associated with sulfur is just hydrogen sulfide that forms when a sample of sulfur is exposed to moisture in the air.  Sulfur dioxide has a bad odor too, but it’s more like the smell of a matchstick just after you strike it.  

What sets hydrogen sulfide apart is that it doesn’t take very much at all to cause a stink.  The odor threshold for H2S is just 0.47 parts-per-billion.  So you can see how it wouldn’t take much to ruin a batch of wine.  Fortunately, this also means it doesn’t take much metabisulfite to counteract the odor, and the sulfur dioxide concentration involved probably isn’t anywhere near enough to be noticed. 

What I think is happening in your grape juice is that it’s been sitting around for a long time, and it’s just begun to spoil.    There were probably some microbes present in the juice in the first place, and the preservative was able to keep them in check.  But over time the sulfur dioxide dissolved in the juice would eventually work its way out of solution and into the atmosphere.  There was probably enough metabisulfite to compensate for this, forming more sulfur dioxide to replenish what was lost to attrition, but it couldn’t last forever.  And every time you open the bottle, you’re exposing the juice to new microbes floating around in the air.  Eventually, you get to a tipping point where there’s enough microbes present to start spoiling the juice, and not enough metabisulfite ions to stop them. 

I don’t know that hydrogen sulfide would be a major byproduct of this process, but again, it doesn’t need to be, because the odor threshold is so low.  I suppose the human sensitivity to H2S odor is something we evolved to recognize food that’s no longer edible.  Even a small amount tells us that something is seriously wrong. The real danger might be the increased microbial activity, or a much higher content of some other byproduct that doesn’t have a distinctive odor, but the rotten egg smell is what throws up the red flag. 

Transferring the juice from one container to another and back again may have accelerated the process, but this was going to happen no matter what.  My advice is to buy grape juice by the quart instead.