Rhubarb (Rheum rhabarbarum) is a species of plant in the family Polygonaceae. It is a herbaceous perennial growing from short, thick rhizomes. It is in the same family as celery. It contains a lot of water & absorbs it as well. When it sits still after being cooked or frozen the plant tends to release the water. So whether you brought home a cooked pie or a frozen pie. If it sat; the water will be released. Simply warm the pie or cook the pie before serving and the water should dissipate. Hope this helps.
If you thawed it out and it is a uncooked pie water is common, then when you cook it the water cooks out.
Because it was previously frozen ! They do that alot !
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