As explained later in the movie, it's not so much his scream than his death that triggers the motherbox.
Circa 2h20
Bruce :
Victor's father activated a Mother Box more than a year ago
when Superman was still alive. Didn't call out to Steppenwolf. None of
the boxes did. Not until...
Diana :
Not until Superman died.
Bruce:
Not until Superman died.
Barry:
It's like they were afraid of him?
Bruce:
Yes, they were afraid of him.
My guess here is that the oscillation is the call to Steppenwolf.
I'm aware it's only a partial 'why', it doesn't fully explain how the motherbox were omniscient enough to understand this scream was a death scream, or at what level the motherboxes were sentient enough to be afraid of superman.