It yellowed
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UV exposure over time
Most epoxies yellow under ultraviolet light, and a piece in a sunny window will show it within months. Formulations with UV inhibitors slow it and do not stop it. For anything that lives in daylight this is a product choice made before the pour.
Heat during cure
A pour that ran hot yellows immediately rather than over time. If the piece was yellow coming out of the mold, this is the cause and it points back at mass and depth.
The product was never clear-rated
General purpose and structural resins are not formulated for optical clarity and some are faintly amber from the start. Using one for a display piece is a specification mistake rather than a technique failure.
Pigment hides some of it, and only some
An opaque piece tolerates a resin that would be unacceptable clear, which is a legitimate way to use cheaper material. A tinted translucent piece still shows the shift.
Set the expectation if you sell it
If a clear piece will change in daylight over years, saying so is better than a complaint later. It is also simply true of the material and buyers who care will respect being told.
Store stock out of light
Both uncured resin and finished stock. A shelf by a window is a slow degradation of inventory you already paid for.
Check it against a reference, not against memory
Yellowing is gradual and the eye adapts, so a piece can shift a long way before anybody notices a step. Keeping an early casting in a drawer, out of light, gives you the comparison that makes the change visible.
If a customer reports it on a recent piece rather than an old one, that is a cure problem rather than age, and the pour log will say whether that batch ran hot.