When you type in recipients of your email, Gmail suggests its recipients (To/Cc/Bcc) from those added to Google Contacts.

All faculty, staff, and secondary groups are already added to your Contacts. If you add someone you frequently communicate with to Contacts, Gmail will be more likely to suggest that person.

Moreover, Gmail automatically adds someone you contact for the first time by default, making people you emailed more likely to appear in the suggestions.