Communication becomes circular when

Communication becomes circular when the decoder in turn becomes an encoder, so both parties keep swapping roles.

As the receiver interprets a message and then replies, the loop of sending and responding closes into a circle.

Absence of feedback would freeze the flow into a one-way line, which is the opposite of a circular process.

A credible source improves persuasion but does not by itself turn a linear flow into a circular one.

A clear channel reduces noise yet still leaves the model linear unless the receiver answers back.

This role reversal is the heart of the Osgood and Schramm circular model of communication.