It was never dissolved
Why Does Juice Separate in the Glass?
Juice is not one liquid. It is water with tiny pieces of fruit floating in it, and floating things settle.
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Step 1 of 5
Dissolved is not the same as mixed.
Sugar comes apart into pieces so small they drift like the water itself and never settle out. Pulp stays as whole fragments of fruit, thousands of times larger, simply carried along.
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Step 2 of 5
Anything floating gets sorted by weight.
Pulp denser than the juice drifts down. Oil pressed from the peel, lighter than juice, drifts up. Leave the glass still and both happen at once, from opposite ends.
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Step 3 of 5
Small pieces only take longer.
The finer the pulp, the slower it falls. A well-strained juice takes a day to separate and a rough one takes a minute, and nothing has changed but the size of the pieces.
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Step 4 of 5
Cartons win by making the pieces too small to matter.
Forcing juice through a fine gap shatters the pulp into fragments so small that the water's own jostling holds them up for weeks. The result stays cloudy on a shelf.
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Step 5 of 5
Shaking it is a real fix, briefly.
A shake genuinely puts everything back into suspension. Then settling restarts the moment you stop, which is why the last glass out of an unshaken carton is the thick one.
The short version
Juice separates because its cloudiness is solid fruit floating rather than dissolved — and anything floating is sorted by weight the moment the glass goes still.
Try it yourself
Pour two glasses from a fresh carton, shake one, and leave both an hour. The unshaken one grows a clear band at the top the other has barely started.