You are using Windows 10 and you have set a specific printer as your default printer in Windows 10 but the default printer changes to other printers once in a while.
In November 2015 Microsoft released a major upgrade for Windows 10 (Build 1511). This major Windows 10 update introduced a new setting to change the default printer in Windows 10. When turned on, the default printer is the last used printer.
A Brother 5450DN Laser Printer is your default printer in Windows 10 and for a special case you need to manually select your printer to a different printer driver. With this setting turned on, your default printer in Windows will not be Brother 5450DN Laser Printer anymore but the last used printer.