Ethical threats

For the scenario below,

The audit partner and the finance director of the client company, have known each other socially for many years and in fact went on holiday together last summer, with their families.
As a result of this friendship, the partner has not spoken to the client about the fee for last year’s audit, 20% of which is still outstanding.

The answer key stated that the threats to objectivity is :
Self-interest threat, Intimidation threat, Familiarity threat

I don’t understand how an intimidation threat may arise. Could you explain sir ?