Well you add a new model to one of your apps, you add a new field to a model, you even change the help test for a model... Any of these changes can cause the need for a migration. How often how you started up your django project locally and it yells at you:
Your models have changes that are not yet reflected in a migration, and so won't be applied.
Run 'manage.py makemigrations' to make new migrations, and then re-run 'manage.py migrate' to apply them.