"Despite his obvious talent and his charming demeanor, the much-admired drummer was another casualty of the rock’n’roll lifestyle that came with Maiden’s growing popularity in the early 1980s. Previously reliable on stage, Burr started having performance trouble on the 1982
Beast On The Road tour. Often hitting the stage while nursing a hangover, just the same as certain other people in the band, Burr started to let his playing suffer. Cardinal sin. Harris wouldn’t tolerate it for long.
Guitarist Adrian Smith recalls the trouble of
“keeping it together for the rest of us when he was having an off night, which he was having more and more as time went by.” And Harris, by then used to band members succumbing to the lifestyle, and worried about getting through the tour, remembers a night when Burr
“spent most of the gig throwing up into a bucket at the side of his kit.”
“It got to everybody, in the end,” says Smith. And so, at the conclusion of the tour in December 1982, Clive Burr was asked to leave the band that had made him famous."
From
http://maidenrevelations.com/2012/11/09/feature-friday-maiden-exits/