I'm not allo, but my allo classmate had a crush on someone while she was dating someone else last year, which kinda confused me because (she's monogamous) if she was dating someone and happy with the relationship, then why would she want to date someone else, and if she wasn't happy with the relationship, why didn't she break up. Not trying to be rude, just a confused aro here!
It may be different for polyamorous people, though, and I'm not quite sure how it works with monogamous people. I personally don't experience romantic attraction at all, so I don't think I can further assist, but as @lovelyleaf said, for some people it's just a natural thing that happens that they don't want to pursue.