A chemical reaction occurs when hops are exposed to ultraviolet light creating a skunky aroma (often called catty in Europe). Skunkiness is very common in green or clear glass bottles but can also occur in brown bottles. Certain varieties of hops are more sensitive to ultraviolet light.
Corrective Action:
Store your hops in a cold dark place; your freezer works best. Bottle in brown bottles and store your beer in a cold dark place.