What is the mother waiting for in the poem, 'To a Daughter Leaving Home' written by Linda Pastan?

From the text:

I kept waiting

for the thud

of your crash as I

sprinted to catch up,

while you grew

smaller, more breakable

with distance,

pumping, pumping

for your life, screaming

with laughter,

the hair flapping

behind you like a

handkerchief waving

goodbye.