King Francis I of France, after being captured by Spanish/Imperial forces at battle of Pavia, had to send his two older sons to Spain to regain his freedom according to Treaty of Madrid. What would happen if sons of French King died not long after arriving to Spain, say they fell ill, so Charles V send them doctors-Spanish doctors at the time were able to kill even healthiest person, so it is easy to imagine that something went wrong.
What happens next? Charles V lost his hostages and would be blamed for their deaths, Francis would hate Habsburgs even more on one hand, OTOH he does not need to care about his sons in Spanish captivity anymore. Charles V would not enforce match between his sister and French King, while Francis, after loss of two sons, would seek for new wife, there is only his youngest son Charles left, so Valois line is in danger.
What happens next? Charles V lost his hostages and would be blamed for their deaths, Francis would hate Habsburgs even more on one hand, OTOH he does not need to care about his sons in Spanish captivity anymore. Charles V would not enforce match between his sister and French King, while Francis, after loss of two sons, would seek for new wife, there is only his youngest son Charles left, so Valois line is in danger.