From DuckDuckGo's &t= page:
Through partnerships with developers and companies, DuckDuckGo has been integrated into many applications. In these partnerships, a portion of DuckDuckGo's advertising revenue is sometimes shared back. To assign advertising revenue and collect anonymous aggregate usage information, developers add a unique "&t=" parameter to searches made through their applications.
They say the results are an "anonymous aggregate" and not for customizing results or tracking users.
Their API docs also suggest apps using the API should include a descriptive
Use a descriptive t parameter, i.e. append &t=nameofapp to your requests.
Sounds like whatever software you are using on your Raspberry Pi is simply using this feature as instructed.