Can you describe in detail what happened, in what order?
But a blind guess, or rather hint: there are two shutters in a Hasselblad.
The one in the lens is timed, and only requires you pressing the release button.
The auxillary shutter in the rear (the thing that shields the film from light when the shutter in the lens is open so you can focus and compose) is timed by your finger pressing the release button.
When you press the button, both shutters open. The shutter in the lens will close after the timer in the lens has finished what it does. The one in the rear slams shut the moment you take the finger off the release button.
Those two events, both shutters closing independently from each other, may be what you noticed.