Hi Rajan,
Assuming that you're talking about SSH here ... this is because the SSH server in AireOS isn't really a full SSH implementation - it's only an SSL wrapper around an internal TELNET session. So your SSH client can't pass the username out of band to the AireOS server ... you have to enter the username inband.
This is fixed in AireOS 8.6 via CSCve45024.
Hth,
Aaron