Had someone demonstrate to me in my 120 the danger of getting a strong gust under the tail by flying backwards at no more than 15 knt. No warning, but wind got under the tail and pitched us nose down just like that. Can't apply rear cyclic - risk = chop the tail - so "wait it out".
Demo over an airfield with adequate height to demonstrate safely so nothing more than a twitching sphincter. It put me off setting down downwind as I reasoned that the same could happen from a tail-wind, and close to ground / buildings I would have no time to recover.
The speed of attitude change looked just like that to me; and he had no room to recover.
Not saying there was not some other cause, just that an on-shore breeze of "less than you might think" might do exactly that.