Programs keep launching on my secondary display - why?

I've had this happen sometimes. 

Normally, Windows keeps track of window positions even when you change secondary monitor (e.g. removing the secondary monitor moves the windows to the main monitor, but if you reconnect again, they will be moved to the secondary where they were originally). Windows even differentiates between each secondary monitor.

While I don't have a direct solution for you, I do have some workarounds. 

PowerToys "Fancy Zones" and "Workspaces" might help mitigate this problem for specific apps that are annoying: 

https://learn.microsoft.com/windows/powertoys 

Hope this helps.