When an organization is deleted that is the associated owner of a memberhip type, then the membership type still exists and one of the individuals with that membership type becomes listed as the Organization.
To recreate on the demo:
1. Create a new membership type and assign an organization (e.g. Company A)
2. Create a membership for an individual contact (e.g. John Smith) into that new membership type
3. Delete the organization (Company A) from the contact database
Now if you view the individuals membership tab, you will see that their membership is still listed. Go to the Membership Type administration menu and view the membership type that had been created, and observe that 'John Smith' is listed as the organization.
Desired behaviour is probably to not allow deletion of organizations that are used in one or more membership types, following a similar principle expressed in:
As a feature, it would be nice to have a listing of membership types associated with an organization. For instance, if you view the membership tab of an organization it would show you Active Memberships, Inactive Memberships, and Membership Types