In OmniFocus > Preferences > Organization Preferences you’ll find a “Clean up inbox items which have” setting.
In your case, it sounds like it’s set to either “A Tag” or “Either a Project or a Tag”. Since actions can only live in either the inbox or a project, they’re moved to the “Miscellaneous” single action list if no project is specified.
If you want actions to stay in the inbox until you assign a project, choose “A Project”. And if you want to enforce adding both a project and tag on all actions before they leave the inbox (my personal preference), choose “Both a Project and a Tag”.
It’s worth noting that this setting syncs across all of your devices. So, for example, if you apply this setting on your Mac it will also take effect on your iPad.