Are they cis guys who are attracted to trans women but don't know how to process that in healthy ways so it comes out in transphobia? Are they just transphobes and they get their jollies out of being straight up sicko creeps? Or something else entirely? I'm just curious as to what makes them tick.
I'm a straight (ish) trans woman, so I have some takes on this. The word chaser gets used in a bunch of ways, including just to mean anyone attracted to trans people. Personally, I'm kinda glad that there are people attracted to me, otherwise I'd be lonely. I would say chasers are anybody wants me, but isn't willing to go to the trouble of treating me like an actual human. There as basically two different types.
Type one are "open minded sex pests". They tend to be the stereotypical married older guy who tries to meet "passable cross dressers" on Grindr and will refer to you with some bizarre porn term from the 80s that no one your age has ever uttered. They don't have to be this, though, they can be any age.
There's a funny scene in a book by a trans author I read once. Character one is trans but stealth, character two is an Asian cis woman complaining about her shitty ex boyfriend. Ch. 2 recounts how she found her ex's extensive asian porn stash, and how gross it made her feel. Ch. 1 immediately jokes "Oh, what kind of chaser is that?", and then realizes she fucked up when Ch. 2 asks what a chaser is.
My point is that there is no difference between "tr*nny chasers" and any other type of man who fetishes a specific group of women, whether it's Asian women, very young women, skinny women, curvy women, or the elusive gamer girl. The reason chasers are considering their own thing is that's trans people are uniquely marginalized, making us easier to exploit, and also because the transphobic general public assumes that chasers must be gay or something.
Type two are those who are repressing their own queerness in some way. They may be bi or gay and want to experiment, but often they are straight up trans themselves and haven't come to terms with it yet. I obviously have a lot more sympathy for this group, but the dynamics can still be kinda toxic because they want to live vicariously through you to some degree. I haven't met anybody like this, but it's a stereotype to the point where there are memes about it. I have had some guys who came out as bi right after meeting me (gee thanks...).
I think there are also slightly different kinds of chasers who go after trans men, but you'd have to ask one of the guys about it.