NetworkManagerでデフォルトの設定がおかしくなったとき
The way to use network manager for default network assignment is by checking the 'Use this connection only for resources on its network' option in editing IPv4 routes on all connections you do NOT want to be the default.
To do this without Network manager simply edit connections that you don't want to be default and add the following at the end of each corresponding file in /etc/NetworkManager/system-connections/YourConnection
never-default=true
this line should be added under IPv4 section.
NOTE: Replace YourConnection with the corresponding Connection name. You can see available options via
ls /etc/NetworkManager/system-connections/