Why does chromosome 13 get an extra copy?

Noone knows: If you mean the birth defect of trisomy 13, noone really knows why that defect is able to survive until birth.Mccoy out!

Error in formation: During formation of the egg , the chromosome (c) pairs that make up the normal cell separate and each egg is supposed to have 1/2 of a pair. If a 13 pair does not separate, three rather than a pair of 13's will form when the egg & sperm join. Having an extra 13 in every cell of the fetus confuses the fetal development. Miscarriage or birth defects often occur.