In the event of a sudden bandwidth spike, quickly open nethogs and find the process responsible. Nethogs reports the PID, user and the path of the program.
Install Nethogs - Ubuntu, Debian, Fedora users get from default repos. CentOS users need Epel
# ubuntu or debian (default repos)
$ sudo apt-get install nethogs
# fedora or centos (from epel)
$ sudo dnf install nethogs -y