Imparfait ou passé composé?

J.S. 

This is a piece of background information, a reason for the ‘ being late ‘ , the event being described. The speaker is not describing an ‘ event ‘ specifically. 

The ‘ washing up ‘ itself may have been a precise event, but the ‘ needing to do so ‘ which is being mentioned here, is relatively indefinite. 

On both counts, imparfait is the tense used in everyday French. 

The links below may help with understanding this usage of imparfait to ‘ describe something ‘ 

 Expressing opinions and describing with the imperfect tense in French (L'Imparfait)

 https://www.frenchtoday.com/blog/french-verb-conjugation/passe-compose-versus-imparfait/