Dry hopping versus oxygen in the secondary fermenter question

[mr_beer]

Because I am not an expert, my inclination is to ensure that beer in the secondary is ?up to the top? of the carboy capacity.  This is because I am concerned about the dreaded oxygen exposure for the beer and the resulting side effects. 

Recently I tried to dry hop an IPA style beer in the secondary.  Lucky for me I had it in a laundry tub because it started to overflow the carboy capacity by a lot. 

So, my conclusion is that something is happening with the beer and the hops during the dry hopping process.

This time around I put the beer in a secondary container that had about 1 gallon of unused headspace.  Lots of stuff happened but the manifestation was that the airlock started bubbling soon after the hop additions.

My question is related to what is inside the carboy ? did the dry hops start fermentation again and the resulting CO2 displaced the oxygen?

Put differently, am I obsessing about the O2 and its consequences?

Any help guidance would be appreciated.

[jomebrew]

What you see is the CO2 exiting the beer and bubbling the airlock. 

The fermented beer in the vessel has about .75 volumes of CO2 in it. Once you add hops, it creates nucleation sites or, more simple, a path for the CO2 to escape the beer. Just like dropping salt into a beer or simply pouting it into a glass.

Yes, you are introducing oxygen but that is both scrubbed by the escaping CO2 and the yeast in the beer.

Hops do have sugars and the yeast will ferment them. So, there can be some activity from that additional fermentation too. It is good to give the beer a diacetyl rest at around 70F for a couple days to encourage the yeast to break down the diacetyl before bottling or kegging.

[Kevin58]

Consider skipping the secondary step. Even the experts like John Palmer who wrote about doing a secondary in the first edition of his book, How to Brew, now say they regret perpetuating the use of a secondary vessel. You can instead dry hop in the primary fermenter and by doing so while there is still a bit of fermentation occurring relieve the risk of oxygenation. I haven't used any of my carboys as a secondary for nearly 10 years.