Spring and summer are ideal times for yeast and bacteria, who may have been mid-ferment when cold winter weather hit, to wake up after a long sleep and begin partying again! I’m sure many of our fellow readers experience this phenomena at some time. If this re-fermentation happens when the wine is in bulk storage, I wouldn’t worry because you have plenty of time for the microbes to settle down (i.e. die off) once they’ve finished eating up whatever food source (sugar, malic acid or something else) that you’ve got in your carboys, providing it doesn’t take so long that the wine gets oxidized. If all goes well, you can rack off the sediment when the wine falls bright; it helps to blanket your containers with carbon dioxide gas to retard oxidation if the fermentation isn’t producing enough of its own. Just don’t bottle up a wine before it’s done “finding itself” or you’ll have carbon dioxide, turbidity, popped corks and off-aromas.
The fact that your older carboys have renewed some kind of fermentation is interesting and points to either a new microbial infection of some kind (see my notes about Brettanomyces above for one possible culprit) or means that you had some residual sugar or malic acid hanging around. Sending your wines out for residual sugar, malic acid as well as microbial plataing analysis is the only way to be sure. White Labs has a new home-plating kit as well as some yeast detection tools that may be of use. And yes, blending wines (again, see above) can indeed trigger re-fermentations. Picture a sweet wine with an inactive population of yeast being mixed with a dry wine that happened to have a few strong yeast cells left in it and presto, you’ve got bubbles.
As to your question about whether or not malolactic bacteria (like Lactobacillus, Pediococcus or Oenococcus) can be airborne, you bet they can be. In fact, I’d bet my Napa mortgage that you’re breathing some in right now. This is one of the most constant battles we winemakers wage — the constant onslaught of ambient (floating in the environment) yeast, bacteria, molds and fungi that live on the surfaces of our equipment, on our grapes in the vineyard and as a result in our wines. This is, after all, how “native” or “wild” fermentations start — because of the yeast cells naturally present in the air. Don’t go all clean-room on us, however. Not even commercial winemakers try to do that because while we can knock down yeast and bacteria cells to acceptable levels, it’s impossible to eradicate them 100%. Burning sulfur sticks in the air of your winery won’t do much — you’d be better off putting some elbow grease into your cleaning and sanitizing regime and investing time in making sure you’ve got adequate free sulfur dioxide levels in your wines during storage.
We all must come to accept, at some point or another, that rather than kill every last microbe in our wineries (which is impossible, even if you burn the place down) all we can do is try to get to know and live with those that we have. It’s important to control the possible food source, the microbes, and preferably both to avoid unplanned fermentations and microbial infections. Good sanitation, inoculation with known microbes and monitoring your critical fermentation parameters (residual sugar, malic acid levels, % ethanol and mg/L VA) will go a long way toward achieving those aims.