Fashion is largely a matter of historical accidents and, being before very recent times barely constricted by any form of 'natural selection', almost anything can happen.
For a military origin of the tricorne revival, Sweden resurrected it in the early 20th C.:
Oddly it looks less archaic, obsolete than the bicorne (still worn recently by the Italian Carabinieri and the Spanish Guardia Civil for instance) so... The military beret nowadays almost universal is the Highlander bonnet of the 17th C., after all.
But 20th C. Sweden lacks the military prestige of Napoleonic France and FPW Prussia, the uniforms of which were so widely copied (specially in Latin America). The 'Scottish' beret gained general popularity because it was worn by 'Free' soldiers of many nationalities during the WW2, and chiefly because of its association with elite troops (commandos, paras). So to spread the tricorne would have to be issued to actually fighting, and rather "glorious" units. Nixon gave a very fanciful 'Ruritanian' uniform with a kind of 'AWI light infantry' cap to the White House Police, but such troop hardly qualifies.
By the times of armies of conscription, such military headgear can spread to the civilian population after a war: almost all French veterans of WWI adopted the beret of the Alps troops, hence the now traditional (but outdated) image of the Frenchman with a beret.
Modified tricornes are still part of (actually, were resurrected for) many female military and police uniforms
Imagine the Duchess of Cambridge (or her sister Pippa) made honorary colonel of the Blues and Royal...
For a civilian origin of a tricorne revival, anything can happen -think of the quasi-hussar costumes of the "70. A series of very popular 'Pirates' movies could do the trick. Otherwise, tricornes are still worn OTL:
- by women in 'traditional' riding hunt dress
(concurrently with the 'postillon' hat and the jockey / riding competition cap : the 'equestrian helmet', which can easily be 'dressed' in a tricorne).
As I remember both 'Breton' (Britton) round hat and the flat-topped 'postillon' hat knew some favor in the '70 - '80, so why not?
- also as part of the compulsory '18th C.' costume of the
cavaleiras and
cavaleiros, the practitioners of the Portuguese riding bull-fighting (
tourada):
(though I doubt such inspiration would be perceived as 'politically correct' in some circles, the PETA among them).
And tricornes are already quite fashionable among some goth / emo circles:
Just have a very popular singer wearing one.
At an earlier date, ca. 1900? The all-new, 'ultra-modern' dress was the bicycle rider outfit. Have it with a tricorne (for the women, as a reference to the traditional riding habit?).