Fails happen for mundane reasons: operational errors, a recalled borrow, or a chain of dependent settlements where one link breaks. Persistent fails in a specific security can also indicate naked shorting.
Regulation SHO requires that fails in threshold securities be closed out within a set window, forcing the broker to buy shares in the market regardless of price. A forced buy-in in a thin, heavily shorted name contributes to the mechanics of a short-squeeze.
Shortened settlement cycles reduce the window in which fails can accumulate but leave less time to fix operational problems. Published fails data is a useful, if noisy, indicator of borrow stress in individual names. See locate-requirement and t-plus-one.
Related: locate-requirement, hard-to-borrow, short-squeeze, t-plus-one, settlement, securities-lending