It's not even about specifically wanting bottom surgery. It's more fundamental than that. You want a partner that will love you independent of your trans status. (I want to say, "in spite of your trans status" here, but that feels too negative a connotation.) But a chaser is interested in you BECAUSE you are trans. That could be a sexual fetish, or it could be someone that's maybe looking to live vicariously through you (a lot of chasers are reppers.) But the term "chaser" is literally that - a type of person that chases after trans people specifically.