How could someone be an accident?

Mary's father, Noel Norman Dinkle, worked in a factory,

 attaching the strings to tea bags.

At show and tell, she told the class he could get

 as many free tea bags as he wanted.

Her favourite tea bag was Earl Grey. She loved saying

 "Earl Grey"

and would like one day to marry someone called Earl Grey.

They would live in a castle in Scotland, have 9 babies, 2 ducks...

and a dog called Kevin.

Noel's hobby was to sit in his shed and drink Baileys lrish Cream

and stuff birds he'd found on the side of the freeway.

Mary wished he'd spend more time with her and less with his dead friends.

She also wished she had some brothers and sisters.

Her mother had told her she was "an accident". 

How could someone be an accident?