Why do you lose a chromosome if you have turner's syndrome?

Nondisjunction. When the cells divide in the ovaries and testes to form eggs and sperm, they go through a process called meiosis where half of the paired chromosomes go to one cell and half go to the other. There is a phenomenon called nondisjunction where the chromosomes segregate unequally, causing an extra chromosome in one cell and the lack of a chromosome in the other.
Its the other way. Losing an x chromosome leads to ts. It happens during early cell division of conception. The cause is not know.