Hmmm, I guess if you really were to really reach for a lore reason, you can look into Ala Mhigo which is where most Hyur Highlanders are concentrated in is a war torn region. I can imagine the Autumn War, a conflict between Gridania and Ala Mhigo. And the civil war that later led to the Garlean invasion. Most highlanders seem highly concentrated in that region, specifically in Ala Mhigo, so it is to say the highlanders we see in other areas of the game have fled the city after it fell according to [1].

"The Autumn War was a military conflict between Gridania and Ala Mhigo in the years 1468-1469 of the Sixth Astral Era, over 100 years before the events of Final Fantasy XIV. Little is revealed about the conflict in-game, but it had important implications for the realm, as it led to the formation of the first Eorzean Alliance, the creation of a balance of power, and the adoption of gil as a universal currency."

And I guess the Civil War turned Imperial invasion

"Twenty years prior to the start of Final Fantasy XIV, Ala Mhigo was embroiled in a civil war against the tyranny of Theodoric, the King of Ruin. The resulting instability led to invasion by Gaius van Baelsar and the XIVth Imperial Legion of Garlemald. Weakened by civil war, the largely Hyuran population of the city was defeated by Garlean invaders. Those who would not submit to imperial rule fled west and south, while those who remained either submitted to Garlean rule or formed the Ala Mhigan Resistance."

With a note

"The fall of Ala Mhigo took place 15 years before the beginning of 1.0, as per quest dialogue (information via an NPC named, Sundhimal, during the quest "Winds of Change"). This information, coupled with the separation of 5 years between 1.0 and Realm Reborn, would imply that Ala Mhigo had fallen 20 years prior the beginning of A Realm Reborn."

I guess it's plausible to say that the men of such an unstable region have decided to shave their eyebrows to look more....intimidating to their enemies???? I hope at least they can make the facepaint look convincing...