It's both.

The eyes have to be defective to be prone to silver eye. However, some dolls develop it sooner, some later, some not at all. So even though you may have a defective eye, you never know when or if it may actually go silver.

Getting water or storing the doll in very humid conditions can make it show up sooner too. So basically although the eyes are defective, it's up to chance if they actually go silver.