Why can Lafayette be considered an

improbable leader to the American Revolution

Because Lafayette was a member of French noble family

The American revolution was seen by the French people as an inspiration that common folk had the power to overthrow the current monarch that ruled over them.

If French people decided to imitate american revolution, Lafayette would be in danger.

But, He still follow his own principle and give his best support for the American revolution. This is why he was considered as improbable leader to the American Revolution