This can happen if the host has a '-' in its name. (Even though this is legal according to RFC 952.) (Tested using Git Bash under Windows 10 using git 2.13.2.)
ssh prompts me for a password for any host that happens to have a '-' in its name. This would seem to be purely a problem with ssh configuration file parsing because adding an alias to ~/.ssh/config (and using that alias in my git remote urls) resolved the problem.
In other words try putting something like the following in your C:/Users/{username}/.ssh/config
Host {a}
User git
Hostname {a-b.domain}
IdentityFile C:/Users/{username}/.ssh/id_rsa
and where you have a remote of the form
origin git@a-b.domain:repo-name.git
change the url to use the following the form
git remote set-url origin git@a:repo-name.git