secondary fermentation in my whiskey barrel

I just brewed 10 gallons of Imperial brown ale. OG was 1.070 and after 2 weeks in primary it was down to my targeted 1.016. I racked half the beer into a secondary carboy and the other half into a freshly dumped 5 gallon oak whiskey barrel, I was not able to completely fill the barrel so I topped it off with CO2 in the head space. 3 weeks later I went to take samples and got a bit of a surprise. The carboy was still sitting at 1.016. So far so good. When I removed the bung from the barrel I heard a "pop" as co2 escaped the bunghole. When I looked into the hole there was some kreusen that was slowly rising and making it's way up to the hole. I recorked it and after a few minutes it had settled down enough for me to take a sample. The SG was down to 1.011. I automatically assumed it had gotten contaminated and had bugs in the barrel (Is this common for whiskey barrels?) but upon tasting it I did not detect any off flavors. I am a certified beer judge but that doesn't mean I pick up every little problem with a beer, however I would think I be able to detect lacto, or brett or anything that would cause a secondary fermentation in a whiskey barrel. Does anyone have any experience with what may have happened here? This is the first time I've used a barrel, so this is all new to me. Thanks in advance.

P.S. the beer in the barrel was very slightly carbonated so that could have affected my hydrometer reading but I would think carbonation would cause a higher reading rather than a lower.

It also occured to me that water from the beer could have escaped through the walls of the barrel since I live in a very dry climate here in NM. but that would not explain the carbonation and CO2 build up in the barrel.