underpitching of yeast; long periods of wort cooling (overnight); contamination from equipment; poor yeast strain; too soon removal (fining) of yeast (before it can reabsorb the diacetyl); high adjunct ratio in wort; low fermentation temperature; premature lagering; any process that stimulates yeast then immediately removes it from suspension; use of contaminated sediment for re-pitching (bacteria often coexist with yeast in the sediment).