Conversation so far:
Who was F. W. de Klerk the vice president for?
For South Africa.
Who was the president during the vice presidency of F. W.  de Klerk?
Nelson Mandela.
What was F. W. de Klerk's relationship with Nelson Mandela like?
F. W. de Klerk's working relationship with Mandela was often strained, with the former finding it difficult adjusting to the fact that he was no longer president.

Question: Why was their relationship strained?
Self-contained rewrite: Why was F. W. de Klerk and Nelson Mandela's relationship strained?