Nothing - gender is a subjective, aesthetic impression. Where there is no convention, no info that "tells" a person the supposed gender of a thing, the human mind naturally tries to see a pattern and believes its own bs.
Like, the name
River has sounds in it like
Richard and
Grover, which a person might have known men named, and maybe their experience is such that when they imagine a river, what comes to mind is a dude they know who leads rafting trips. Or say,
Rain sounds like
Reina and
Lorraine... in the absence of sexed bearers, deciding gender means generating a response to something about the name, and it's going to be personal and subjective.
It's not different from asking what color is a name, or which names sound extrovert or introvert, or which names are night names and which are day names... summer and winter names...
Everyone's impression of name gender is valid just like everyone's favorite color is valid.
It's only when a name is predominantly used to name persons of only one sex, that its gender becomes a convention and based on real information, not a subjective aesthetic judgment.
- mirfak