What is the main reason for tissue necrosis sometimes seen after a local anesthetic injection?

Localized tissue necrosis after an LA injection (most classically seen on the hard palate, where collateral blood supply is limited) results from prolonged vasoconstrictor-induced ischemia — particularly when a concentrated bolus of vasoconstrictor-containing anesthetic is deposited in an area with limited collateral circulation, causing prolonged local blood flow reduction severe enough to cause tissue damage.