Just reasoning by way of analogy to Android, which lets you choose what calendaring app to use when you add an event from a different app to a calendar. In addition to choosing the calendaring app you want, it asks you if you want to always use that app, or just this once. If you select "always", then you no longer get asked which calendaring app you want to use. If you later want to use a different one, then you need to go into the settings & reset the default app for calendaring. Maybe iOS has something similar and somehow it got set to always save events to your native calendar instead of Outlook? Maybe uninstall & reinstall Outlook & Tripit, and then you'll get asked again which calendar?