So, I wondered if any of you folk would have an explanation for a phenomenon I just witnessed.
So I have this opened bottle of wine in my fridge, it's not fizzy wine. for whatever reason, I've left it there unfinished for some weeks

I wanted to have a glass with lunch, so I took it out and took the cork out (it's a reused port bottle cork, since the original one could not be wedged back in place) and I got a surprise. When I took it out, the cork came out more forcefully than expected, and with a very loud pop... like gas pressure had built up inside the bottle. except I have no idea where said buildup could have come from, since it's a still wine.

Also, I don't think it started fermenting again, as a) it's a dry wine, so no more sugar to ferment, and b) the taste did not change noticeably.

Check the local meteorology, perhaps. High atmospheric pressure when you put the opened bottle in the fridge and lower atmospheric pressure when you removed the cork.

Or, indeed, vice versa.

Or quite possibly the air in the bottle contracted as it cooled, creating a pressure difference. I've found this before with Tupperware boxes containing food that have been placed in the fridge while still warm. The lids are drawn inwards as the pressure drops inside the container.

A simple test for fermentation is to slip a balloon over the mouth of the bottle. If the ballon tries to inflate you have fermentation, which could be for either alcohol or vinegar. A mew strain of yeast could have entered the opened bottle and is acting on what sugars are still present, and even dry wines have a little left over sugars the parent yeast might not be able to convert to alcohol.

RS

I'm just trying to get my head around the idea that a bottle of wine can end up back in the fridge once opened.

It's rosé, I must admit that a bottle of red would likely have lasted MUCH less.