Surely there would be a number of different motivations and it can be difficult to tell when an interest in something is genuinely "surface-level" unless you have that interest yourself, so I don't know if I'd label this as a single broad phenomenon. I also don't know whether I'd label "suddenly getting into something new" as a guy thing as opposed to just something that I guess stands out more when guys do it. Heck, who knows if it's even something they "never showed interest in before" versus just something you never noticed them taking interest in before, or they just didn't tell you in particular, or they did tell you and you just forgot? I'm thinking about all the times when people are like "you never showed signs of being trans", but actually you showed plenty, it's just that the possibility of being trans is so far outside of the cis brain that even the eggiest of behavior goes right over their heads. It can be the same for interests/hobbies, especially if people are worried about being judged as cringe for it.

Not to ascribe motivations to people I don't know, but if I heard about somebody in my life suddenly converting from atheism to Islam without consulting anybody in their life about it first, then I'd have to wonder if the person was experiencing psychosis. This isn't to say that people can't genuinely convert from atheism to a religion, but when that conversion is particularly sudden and accompanied by erratic and irresponsible behavior such as converting without telling your partner of ten years, then the total change of worldview to start believing in supernatural forces becomes a bit concerning. For people who take a sudden interest in the Roman Empire, my first thought would be that it's a cryptofascist red flag, that the interest is surface-level because it's more of an aesthetic-based dogwhistle than anything, and actual scholarly knowledge of the Roman Empire would be counterproductive to the fascist worldview. Which isn't to say that it's impossible to take an interest in the Roman Empire in a non-fashy way, just that I'd approach with much caution. As for otherkin, then that's a subculture that skews very autistic, and I would know a thing or two about letting my life suddenly get consumed by a new autistic interest.

All of this based on people I've known who converted to Islam, got into the Roman Empire, or were otherkin, although a few anecdotal experiences aren't enough to speak for people as a whole. As I'm saying, there are many different motivations behind why people might suddenly change so much, so we should try to be less judgmental on the whole. Even in the cases where it genuinely is just somebody deciding to become a Different Cooler Guy for the sake of being a Different Cooler Guy, then should we really be judging people for finding ways to experiment with their identity? Maybe you'll feel some second-hand embarrassment from seeing somebody's seventieth Guy 2.0 debut, but the state of being alive in general is a phase, so why should we devalue the sub-phases that life contains? The problem with suddenly deciding to wear a leather jacket is the leather part, not the change in wardrobe. To be cringe is to be free!

I dunno.

You should watch SPARKLE ON RAVEN: The Life of DrillGirl; it is a very good show. {Cēterum|Furthermore} {cēnseō|I think} {iūs auctōris|copyright} {esse dēlendum|must be destroyed}.