Why does Visual Studio 2017 open Windows "For developers" settings on startup?

I had the exact same problem when opening a Xamarin project. (Maybe it's related to the configuration request of the Xamarin Mac Agent)

I've found that enabling the developer mode on the settings stop the panel to popup everytime.

It shows up because of the UWP workload (along with a notification in Visual Studio that you need a developer license to develop UWP apps):

https://docs.microsoft.com/en-us/windows/uwp/get-started/enable-your-device-for-development

However, if you are writing software with Visual Studio on a computer for first time, you will need to enable Developer Mode on both the development PC, and on any devices you'll use to test your code. Opening a UWP project when Developer Mode is not enabled will either open the For developers settings page, or cause this dialog to appear in Visual Studio:

• Developer mode lets you sideload apps, and also run apps from Visual Studio in debug mode.

Either switching to developer mode or modifying your Visual Studio installation to not include UWP will stop it from happening.