The only explanation I'm aware we've officially gotten is a quote that allegedly came from a 2014 lore panel:
Quote Originally Posted by 2014 Lore Panel
Q: Why don't male Highlanders have eyebrows?

MCKF: There is a reason. Highlanders are a very violent group of people. Their clan is always warring amongst each other and against the Midlanders as well back in their homelands. They like battle. They fight and the women fight and their children fight, because that's their culture. When you have this culture of everybody being able to fight, it's all about getting the one-up on your opponent. Everyone's good with a sword or a club or whatever, so you need something extra. Everyone expects their opponent to have eyebrows. So if someone goes into battle and your opponent doesn't have eyebrows, it psychs you out and your head gets chopped off. Different tribes have their different rituals but one of those rituals is shaving off the eyebrows to put fear into your opponent.

Q: So they do have eyebrows, they just shave them off?

MCKF: Yeah, it's not a genetic thing. They shave them off or they give themselves scars or pull out their teeth. Things like that to make them look more imposing.
I say allegedly because I have often found this quote cited but I've yet to see any footage of Koji Fox saying this himself. I pulled the quote from this thread on the official lore forums. I've also found the post where the question may have originally been posed but was unable to find it in the recording of the lore panel linked at the end of the thread. I also didn't hear it in the subsequent lorecast interview with MCKF linked later in the thread. If anyone can find footage of this quote I'd be interested to see it.

Quote Originally Posted by Astronis View Post
I say it's probably a racial trait because male Highlander Humes don't have eyebrows either. If *they* are shaving their eyebrows to appear more intimidating, the cultural roots are Sundering-deep. Either way--there *is* a reason to stick to it, but there is *also* a reason to give people options.
The practice can't be sundering deep because the two most prominent highlander men in the last two expansions both had eyebrows. When writing a story I'd expect the writers to use the characters from a given clan or race players will spend the most time looking at to represent that group. The humes of the first not having eyebrows comes off to me as the developers being constrained by the character creator when making NPCs.