An interesting discussion for a guy like me. I've dry hopped four of my batches now (all IPA's) and never experienced this phenomenon (that I noticed). The science behind your explaination makes sense to me. However, an airlock that bubbles every 40 seconds seems like a lot if something like this is occurring. It makes me lean towards the side of fermentation is still happening for whatever reason.
I've got a high gravity Imperial IPA I'm planning to dry hop this weekend (I dry hop in primary). I think I'll try and pay closer attention.
Thoughts?
Keep in mind that bubbles are not a reliable indicator of fermentation. The only way to really know what's going on is to take a gravity reading.
The outgassing from hops won't necessarily happen every time. It depends on temp, how much CO2 is in the beer, and the phase of the moon.