Why do my stored cashews smell like fish?
I believe the fat in your cashews is becoming rancid. That would most likely be caused by oxidation. Exposure to air is the chief culprit. However, exposure to light or heat could also accelerate this.

I'd wager you got a jar of nuts that wasn't quite sealed. Have you had this problem before with other jars of these nuts? 1-2 weeks is an abnormally short period of time for this problem to occur in a sealed container.

It's also possible that the nuts are old. Have you checked the "best by" date?

Freezing does not stop oxidation. It would ensure a relatively cool and dark storage place for them, though.

The only way to stop oxidation is to prevent air from touching the food.

Equipped with a new jar with a tight seal and a cool, dark storage place... I think you'll do just fine in subsequent cashew ventures.