FTP disconnecting

This is a odd one. I can be downloading files but when I attempt to up load a file to a different directory on the same site FTP disconnects and attempt to reconnect. Meanwhile both upload and downloads continue normally. The question is if the FTP says it is disconnected and attempting to reconnect then how come both transfers continue? Is this a bug or has FTP limitations?

The FTP site you're using probably limits you to only one connection.

The FTP log should have mote details about what's happening.