It stopped because when you opened it you released the pent up co2, so now it doesn't need to VENT. An airlock is just a valve, a vent to release excess co2, it's not a magic fermentation gauge.
Stopping or starting airlocks DO NOT mean fermentation has started or stopped, just that the airlock has.
Your gravity reading proved you had fermentation.
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.
Fermentation is not always "dynamic," just because you don't SEE anything happening, doesn't mean that any-thing's wrong,, and also doesn't mean that the yeast are still not working diligently away, doing what they've been doing for over 4,000 years.
That's why you need to take a gravity reading to know how your fermentation is going, NOT go by airlocks, or size of krausen, or a calendar, the horoscope or the phases of the moon (those things in my mind are equally accurate).
Step away from the fermenter, everything is fine.