This is the 2994th most frequent German word.
auslöser
Trigger; catalyst; cause.
In this context, 'Auslöser' refers to the cause or trigger of her joyful reaction.
Der Auslöser für ihre Freude war die überraschende Nachricht.
The trigger for her joy was the surprising news.
Here, 'Auslöser' denotes the physical mechanism triggering the photo capture, such as a camera device.
Ein Plötzlicher Lichtstrahl war der Auslöser für das Foto.
A sudden beam of light was the trigger for the photo.
In this usage, 'Auslöser' signifies a contributing factor or cause initiating a state of insomnia.