Haddon, 22, apparently jumped from a communications tower behind the main stage of the Pukkelpop in Hasselt festival Friday night.
The London-based trio had performed at the three-day event and was scheduled for an appearance in Austria Saturday before beginning a tour of Britain in the fall, The Daily Telegraph reported.
There were no details on what drove Haddon to jump; however the band and Belgian authorities said they considered his death to be a suicide.
The Telegraph said Haddon's death came the day after Michael Been, formerly with the 1980s band The Call, died at the same festival of a heart attack. The 60-year-old singer was working on the sound crew of his son's group Black Rebel Motorcycle Club.
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