Excerpts from Books and Wikipedia
"The upshot was that Henry IV, although popular with his people, was vulnerable to challenge. . . . A Welsh farmer named Owain Glyndwr took advantage of Henry's insecure crown and called his countrymen to follow him to independence. . . . And then Henry IV began to make missteps. . . . Edmund Mortimer, who (despite being deprived of his putative crown) had been loyally fighting for the English cause against the Welsh, was taken prisoner in a battle with Owain Glyndwr. Mortimer's brother-in-law Henry Percy, who had been leading the resistance to the Scots, offered to ransom him out of Owain Glyndwr's hands. But Henry IV refused to allow it. This removed Edmund Mortimer, his potential rival, from the English scene. But both Mortimer and Henry Percy, who until this point had been supporters of Henry IV, were indignant. Owain Glyndwr seized on the indignation. . . . Henry IV had accidentally turned the Welsh revolt into a civil war, and the revolt he had created boiled along for another decade. Henry Percy, nicknamed 'Hotspur' because of his tendency to act first and think later, was killed almost immediately;" And the revolt continued. [Bauer: Renaissance World, p. 620-1]