I believe that shares become ineligible for shorting if the owner places a trade (e.g. GTC) with them (perhaps even at a high price).
With disclosed Mutual fund and institutional ownership at over 7%, they could also impact the market if shares had orders placed on them or their broker was advised to remove them from the pool of loanable securities.
Also, I believe that retirement accounts are not permitted to loan securities for shorting purposes.
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