On my wh-1000xm2, wh-1000xm3, wf-1000xm4 and lastly wf-1000xm5, there is a quite high frequency pitch (usually coincides with some public transport beeps, and some accidental squeaking of doors) that toggles ANC to transparent mode automatically. I remember reading something about this on Sony's support website.
My XM4's always do that at the beeps from the cash register, although I always attributed that to their volume rather than frequency. My theory was that they refuse to produce sound loud enough to cancel the beeps for safety reasons.
I suspect as much too. If there's a failure to match the noise perfectly then the headphones are just going be be blasting a loud sound into your ears. And if it matches the frequency correctly but lines up with the sound instead of being out of phase, then it's acting as an amplifier!
Also triggered by baby screams unfortunately
An evolutionary adaptation to ensure parents wearing headphones don't ignore their babies.
We don't ignore him. I wear headphones while soothing him.
I have a new idea for a bike horn.
Please do not duct tape a baby to your handle bars.