Linked from
The 16 pages that link to Outlier, each with the reason it gives.
CorrelationRelated: A few extreme observations can substantially change Pearson correlation.
ResidualRelated: A large residual can flag an observation for closer investigation.
Box plotRelated: Many box plots mark observations beyond the whiskers as outliers.
Stem-and-leaf displayRelated: Isolated leaves can make unusually extreme observations visible.