Why does a watery separation of sour cream occur only after a portion has been taken from the container?

Because you've created a well, a lower point for the liquid to settle. It's the same idea as when you dig a hole in the ground and water seeps in to it. You've just created somewhere for the liquid to settle. The liquid is whey, which is protein, so if you pour it off you are throwing away the protein and some of the nutritional value.

It's just Whey. It's actually just a part of the cream's moisture so just stir it back in and use. Never throw it out, unless you want thicker sour cream (for dip or something)

normal and OK it is called whey you can choose to toss it or mix it back in it simply the water as the curd squeezes the "whey" out

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air comes in, and when it makes contact with the cream, it produces this watery separtaion.

just stir it up it, just separates . store it up side down it will keep longer also good for cottage cheese.