Help diagnosing an off flavor

I brewed this recipe about 6 weeks ago:
Recipe Specifications
--------------------------
Batch Size: 5.50 gal
Boil Size: 7.02 gal
Estimated OG: 1.051 SG
Estimated Color: 14.6 SRM
Estimated IBU: 43.9 IBU
Brewhouse Efficiency: 70.00 %
Boil Time: 60 Minutes

Ingredients:
------------
8.67 lb Pale Malt, Maris Otter
0.75 lb Crystal Malt - 30L
0.50 lb Aromatic Malt
0.50 lb Munich Malt
0.25 lb Chocolate Malt
0.75 oz Magnum (60 min)
1.00 oz EKG (Dry Hop)
0.50 oz Styrian Goldings (10 min)
0.25 oz EKG (5 min)
1 Pkgs White Labs #WLP007 (english ale)

The brew day went well and hit most of my numbers within a reasonable degree of tolerance. I bottled after three weeks in primary, one week in secondary for the dry hop and two weeks for bottle conditioning. Yes, this may be a bit early to be opening the bottles, but it is an ale and the carbonation level is good.

I am getting a strong, strong flavor of dirty, moist cardboard (820, 841 and 842 on the BJCP flavor wheel). I am trying to figure out what I did to cause this.

The wet cardboard flavor would lead me in the direction of HSA. I have used the same process for moving the wort from the MLT to the boil kettle for many other batches and not had this problem. I am still willing to entertain the idea, but I need to explain why only this beer has this problem and no others.

The EKG were leaf hops; this is my first time using them. Could these off flavors be the result of mishandling the leaf hops?

I have been researching but I cannot find any infection that usually causes these kinds of flavors.

Help?

I would probably say oxidation over HSA. When you transfer from carboy to bottle then you might have introduced some o2. That will give you a cardboard flavor.

I think you have two "maybes" here from me! The first is that I think that EKG hops do taste a bit "earthy" so that may be where you get the dirty taste from. That should improve a bit as the beer ages. Dry hopping with an ounce of them for a week might give you that flavor.

The second thought I had was that HSA probably isn't the issue, but maybe oxidation in either the transfer to secondary or to the bottle. My understanding, though, is that usually oxidation takes quite a while to give the "wet cardboard" flavor. Is it possible that you accidently aerated in one of the transfers, or when bottling?

It still could be just young beer, though. It definitely is still young, and at least the dirty flavor might get better in a couple of weeks.

If it's oxidation, maybe there was too much head space in the secondary? Being on the primary for three weeks, there will be very very little fermentation activity to produce CO2 to displace the oxygen, and as I understand it lots of head space will make it even more likely that the surface area of the beer will be protected. Was there any splashing when transferring to the secondary?