Why is Arcadia’s Barrel Aged Shipwreck Porter sour this year?

Many people have observed that Arcadia’s Barrel Aged Shipwreck Porter tastes a bit sour this year.  Mike Ricken in the Michigan Craft Beer Enthusiasts Facebook Group noted this, and asked Arcadia about it.

From Arcadia:

Shipwreck is unique and this year especially. We age our beers much longer in barrels than normal breweries. This years batch was a blend of 3, 2, and 1 year old barrels. The youngest being 16 months. When hops are aged in barrels for this amount of time they start to change flavor. Is the sour note a tart cherry? This is the common off flavor. Also with prolonged aging the risk for “infection” greatly increases. We have tested extensively and have not found any abnormal bacteria, but our palettes have picked up the slight tartness too. Barrel aging is a strange science and we hope our beers come out as similar year to year as possible, but that is near impossible.