From wikipedia:
A fief (/fiːf/; Latin: feudum) was the central element of feudalism and consisted of heritable property or rights granted by an overlord to a vassal who held it in fealty (or "in fee") in return for a form of feudal allegiance and service, usually given by the personal ceremonies of homage and fealty.
In feudalism, a fief generally passes to the son of the previous ruler.
In practice, kings would break this custom from time to time, and grant the fief to someone who was not the heir of the previous lord. I realize that taking lands away from a powerful lord is likely to start a civil war, and would only be done for a good reason, or in extraordinary circumstances. What were these circumstances?
When and why did historical kings grant lands and titles to someone new?
Good answers will cite their sources and be supported by specific examples.