Got Download Traffic While Uploading With High Speed

When I upload my backup using ftp to my backup server, my upload speed get to 1.5Gb/s which is good! **BUT** I see strange things on download traffic!! When my upload speed get bigger than 1Gb/s I see download traffic upto 10Mb/s. I don't know whats really going on here!? there is no app which cause this download traffic and they start together when ftp start tranfering! I use nload to see traffic Any explanation?