[...]
A lender of a stock holds all the cards. At any time
after he has lent the stock, he can call it back in;
the borrower has three days to return it. Marketmakers
who carry positions overnight in the stocks they
"make" have been known to pull back their stock and
force buy-ins. The occasional mutual fund that lends
shares temporarily does this as well.

The short-seller isn't the only victim here. Squeezing
the short drives up prices, creating volume and upward
action that can attract momentum players. But once the
squeeze is over, there's nothing to hold up the price.
Moreover, eliminating short-sellers makes it easier to
drive up the price of an already overvalued stock.

Corporate executives of heavily shorted stocks also
play this game. First they put their considerable
insider holdings into their margin accounts, making
them available for lending by the firm's stock loan
department. Shortly after these executives make their
stock available for lending, it often happens that
they remove their holdings from the brokerage firm. Or
they move the position into the cash account. Both
actions force buy-ins. Result: more volatility,
volatility that has absolutely nothing to do with
fundamentals. 
[...]