Women's BMX: Bailey Aus finished 4th in 1st race, but when lined up for 2nd race listed DNF in 1st?

Question

Women's BMX: Bailey Aus finished 4th in 1st race, but when they lined up for 2nd race she was listed DNF in 1st race. Why?

Answer 1

I saw that too, but I don't know why.

Answer 2

for Olympic lovers

Answer 3

After being placed 10th in the seeding phase, Tanya Bailey did not finish in Run 1 and Run 2 of the semi-finals, and was 6th in Run 3 - therefore the aggregate was not good enough for qualification to the final.