In Windows 10 1511 there is a new feature which is enabled by default, “Let Windows Manage my default printer“. This setting will make the last printer you used the default printer.
Update: In Windows 10 1607 there is a new user group policy setting to turn this feature of called “Turn off Windows default printer management” under Control Panel\Printers
At many customers this is not a wanted scenario so here is the registry key you need to change to turn it off.
HKEY_Current_User\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows\LegacyDefaultPrinterMode Dword: 0x00000001
Easiest way is to use a User Group Policy preference with the following settings:
When searching for registry settings I tend to use Regshot to find them, easy to use, nothing to install. Works great! http://sourceforge.net/projects/regshot/