• Too extensive dilution, if possible keep protein concentration > 1 mg/ml
  
  
• Exposure to degradative enzymes
  
  
• Bacterial and fungal growth
  
  
• Presence of oxygen, heavy metals
  
  
• Change in physical conditions, like freezing/thawing
  
  
• Vigorous stirring or vortexing
  
  
• Extreme pH or temperature
  
  
• Organic solvents
  
  

Good reference about protein aggregation:
Anal Biochem. 2003 May 15;316(2):223-31.
Detection and prevention of protein aggregation before, during, and after purification. Bondos SE, Bicknell A.