Here’s what I’ve found. If you want an app to open on your secondary monitor by default do the following:
1. Open the application.
2. Re-size the window so that it is not maximized or minimized.
3. Move the window to the monitor you want it to open on by default.
4. Close the application. Do not re-size prior to closing.
5. Open the application.
It should open on the monitor you just moved it to and closed it on.
6. Maximize the window.
The application will now open on this monitor by default. If you want to change it to another monitor, just follow steps 1-6 again.