What exactly breaks the Hypnotic Pattern' stupor?

[diplomancer]

So, the spell states "The spell ends for an affected creature if it takes any damage or if someone else uses an action to shake the creature out of its stupor."

But suppose all of the enemies are affected by it ( a distinct possibility if cast from an Instrument of the Bards). Could you apply manacles or tie up all of them without breaking the spell? Or would doing so be considered the equivalent of shaking the creature out of its stupor?

[NNescio]

So, the spell states "The spell ends for an affected creature if it takes any damage or if someone else uses an action to shake the creature out of its stupor."

But suppose all of the enemies are affected by it ( a distinct possibility if cast from an Instrument of the Bards). Could you apply manacles or tie up all of them without breaking the spell? Or would doing so be considered the equivalent of shaking the creature out of its stupor?

Yes, you can manacle or tie them up (or grapple them, even) without breaking the spell. No, it doesn't count as shaking the creature out of its stupor. You need to take a deliberate action to do so.

[Keravath]

Hypnotic Pattern, especially on a bard with an Instrument of the Bards, can and does completely change the nature of an encounter at times. This is part of the fun. :)

In the case you mentioned, as long as the restraints were applied without undue force, which is up to the DM to decide, you should be able to restrain all of the creatures affected assuming that you can apply the bonds to all in less than one minute. Only damage or shaking them awake (or something equivalent determined by the DM) breaks the hypnotic pattern.