this is normal and wanted behaviour.
During update-process the board checks for files changed or missing and fetches them.
You can decide to download them as a packed file and upload yourself or to fetch them by remote-ftp.
For the second case it is needed to put in your ftp pass to allow automated access.
I got a debug message during update yesterday, which is reported here
My headers were sent from this message and I couldn't start the file download.
So I decided to fetch the missing files by remote ftp and it worked fine.
The other method also worked from 3.1.0 to 3.1.1.