What genetic abnormality causes Down Syndrome? How does this usually happen?

An extra chromosome, also known as trisomy can lead to down syndrome.

The most common form of Down syndrome is known as trisomy 21, a condition where individuals have 47 chromosomes in each cell instead of 46. Trisomy 21 is caused by an error in cell division called nondisjunction. This leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception.