Because
you disturbed the integrity of the co2 in the fermenter when you moved it.
That's really it....
Usually the simplest reason is the right one.
If you think of an airlock as it is intended to be thought of- as a valve to release excess co2, and NOT a magic fementation gauge, things like this won't mean anything to you...
It sat there not needing to bubble, forming it's co2 layer but not enough to NEED to go out the airlock, you're airlock is not a magic fermentation gauge, it's a vent, a valve to release EXCESS co2 to keep your beer off the ceiling, nothing more. It did need to release any excess co2, or it was getting out somewhere else, but more than likely it was just sitting in a stasis- it was sitting in a nice blanket on top of your beer.
THEN you moved it....so now it's doing what it's supposed to do.....VENTING gas....
When you start thinking in terms of the real purpose of an airlock, as a vent it's not too hard to fathom why bubble or they don't, why the start and stop for reasons other than fermentation.
The cat can brush against your fermenter and cause the airlock to stop or start bubbling, changes in temp or atmosphere can cause an airlock to start or stop bubbling, but that doesn't mean the beer is or is not fermenting (that's why a hydrometer is the only true way to know what's going on. You could also have kicked up the yeast which kisktarted fermentation, but how can you distinguish the why, just by looking at bubbling?)
You can nudge the fermenter and a bunch of co2 that was trapped in the trub at the bottom of the fermenter can decide to break the surface and change the amount in the headspace, as as can opening the lid like you did.
When you're dealing with a volume of gas in a set space (the head space) with a vent on top, sometimes it's going to release the gas....or it's not. It's going to do it because of fermentation, or because of agitation.....Gas expands, it contracts, it moves...it's fluid, and it has a way out if it needs it.
You said it yourself, you disturbed it.....All it takes is a little common sense to realize that that's gonna change the nice quiet situation the co2 layer was in in prior to you messing with it.
I mean I know that fermentation is a new thing to a lot of people and when starting something out we're so scared that we often forget to look at the obvious- when you move a volume of liquid, you're also moving whatever gas was around it, whether it's the air that's around us or in the case it's mostly co2 in the fermenter, and the conditions prior to doing it, in this case no bubbling is going to change. When you move your arm, do you not feel a little breeze? A little disturbance in the integrity of the air in the room? Why would it be any different inside the fermenter?
