Reason one: Leafpool's business collapsed.
Disclaimer: I don't own the warrior cats.

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Leafpool once had a business company which trained a young cat from each clan to fulfill their dreams of becoming a medicine cat.
Leafpool, herself trained her younger clan mates from Thunderclan. First it was Hollyleaf, and then Jayfeather.
But nothing could prepare her for the shock when she checked her company's bank account on a bright new leaf day.
Her business had no mice left to pay for the medicine cat instructors, and this caused the medicine cat instructors to go on strikes over their wages.
She had to close her business and sell it to Blackstar, who turned it into a baker shop called the shadowed pies.

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Thus, Leafpool paid all three of the instructors, with the mice money from the sale of her company.
She sighed as Squirrelflight's, Sorreltail's, Ferncloud's and even her mother's, Sandstorm, businesses became busy.
But it was not doom and gloom for our favourite light brown tabby she-cat.
Leafpool had become the four businesses' receptionist. She drank coffee and spoke on the phones all day.
While Jayfeather faced injures after injures from careless warriors; who managed to pick up thorns, caused a fight with Shadowclan warriors and did other idiotic things warriors would do (although it was mainly the younger warriors as the senior warriors were too sensible!).

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(A/N: Next up is the second reason! Edited on the 05/03/2015)