On Windows 10, if you turn on Avast passive mode, it activates Windows Defender. Is there a way to prevent WD from starting up?
Well, that’s the way WD works. There might be tweaks out there, but I don’t know about them.
Not so much the way WD works but the way Win10 Windows Defender Security Centre works, when you install another Antivirus, it disables WD. By effectively disabling Avast there is no active antivirus defence so it has enabled WD again. The point of the Avast passive mode was to allow for the user to run another on-access antivirus at the same time to avoid conflict.
There is nothing in Avast to change that, but as Asyn said there might be an OS tweak out there.
Right. It is not an Avast feature whether or not is WD enabled in the absence of a 3rd party AV.
But I am still interested to hear about those OS tweaks.
Feel free to wait for a reply or just “google” it. 
You can disable Defender via Group Policy - https://docs.microsoft.com/en-us/windows/threat-protection/windows-defender-antivirus/use-group-policy-windows-defender-antivirus