One possibility might be the ‘mouth opening’ reflex is elicited by the ‘emotion’ of surprise.

It occurred to me that the other time I experienced that reflex was when I accidentally dropped my laptop. I was able to bend over and catch it, but my jaw tried to drop. There was an element of surprise in that incident, too.

I could see what it would be useful to communicate surprise to our fellow primates but of course I still haven’t been able to explain why the ‘gesture’ that communicates surprise is ‘opening of the mouth’.

So now I looked up what Darwin said and what I had read a long time ago and half-digested and half-remembered (in the Expressions of the Emotions in Man and Animals) – Chapter 12 is titled SURPRISE – ASTONISHMENT – FEAR – HORROR

It begins: Attention, if sudden and close, graduates into surprise; and this into astonishment; and this into stupefied amazement. The latter frame of mind is closely akin to terror. Attention is shown by the eyebrows being slightly raised; and as this state increases into surprise they are raised to a much greater extent, with the eyes and mouth widely open…The degree to which the eyes and mouth are opened corresponds with the degree of surprise felt …’

Darwin explains opening of the eyes (wide open) as part of an effort to increase visual awareness but then writes ” The cause of the mouth being opened when astonisment is felt, is a much more complex affair, and several causes apparently concur in leading to this movement…’

He seems to settle on the effect of opening the mouth on respiration – 1) ‘when we wish to listen intently to any sound, we either stop breathing or breathe as quietly as possible, by opening our mouths , at the same time keeping our bodies motionless’

2) he talks about our jaw dropping when we are amazed (more as a byproduct of relaxing jaw muscles) and sort of explains why the jaw muscles relax (but not that clearly to me)

3) then, ‘There is still another and highly effective cause, leading to the mouth being opened, when we are astonished, and more especially when we are suddenly startled. We can draw a full and deep inspiration much more easily through the widely open mouth than through the nostrils….[in preparation for] great exertion….’

Not sure how many of these speculations still hold up, but Darwin certainly was thorough..