Have you ever noticed that your AirPods Pro sometimes automatically switched from Transparency mode towards that of active noise reduction? Before returning to Transparency mode, still without your intervention.
There is no setting to enable this behavior that multiple users have stated in different situations. All of them had in common the sudden intrusion of a particularly loud noise: vehicle siren, whistle, screaming child, blowing wind, blowing car, etc.
This is apparently a hidden function of the AirPods Pro which, in this kind of situation where the Transparency mode exacerbates the external sound, will try to compensate for it by activating the active noise reduction themselves. One of the users even said he noted it on only one of his headphones, the one on the side where the noise was coming from.
Among those of us who have Pro, few use Transparency mode or only occasionally. It is therefore difficult to confirm the observations described above. If this has happened to you before, feel free to describe the background in the comments.