John de Mowbray started the barons rebellion in 1320. From the small incident of John seizing control of Swansea Castle, the barons' growing impatience with Edward II grew into direct action.
They didn't like the king's favourite, Hugh le Despenser the Younger, having such control over the king and getting away with his land grabbing. And they didn't like the king trying to assert his authority over the Welsh Marches, where the barons had had free reign ever since William the Conqueror. The king's interference in Hugh's dispute with John was the last straw.
At first, the barons were successful, and the king was forced to send Hugh and his father into exile. But the king wanted his favourite back and launched a fresh campaign, which ended with the king defeating the barons at Boroughbridge. Many of the rebels were executed, including the king's own cousin, Thomas of Lancaster and John de Mowbray. Because he started it all, the king was so angry with John that he refused to allow John's body to be taken down from the gibbet for years! It hung in chains in York until local friars eventually persuaded the king to let them bury it.
For more details, see my book, Alina, The White Lady of Oystermouth, coming out in print and ebook at Easter.