I can't tell you exactly what might be the cause. But the only experience I have had with this kind of thing is a dog I know very well, a neighbour's dog. He has a benign tumor on his eyelid, and it is pushing his eyelid out of shape, so his lower eyelid droops slightly too.
If you can't see anything like a tumor or swelling, and if there is no obvious redness or irritation in the eye, then of course, it might not be the same cause.
But you could get this checked with your vet?
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