Unfortunately it's very likely they're being borrowed from a variety of sources, from unsuspecting margin accounts to (yes, it's possible) large shareholders even if they're restricted shares. The securities lending industry (search) is huge and mostly under the radar.

Here's eTrade, for ex., boasting of their lending capabilities. I can't imagine that I was partly responsible, but the URL below no longer works after C&P'ing the info a couple of times on iHump. It may have been moved to another part of eTrade's Capital Markets' site. Anyway, the info is out there if ya dig. Freakin' frightening to see how easily the playing field can be tilted. And, yes, they lend Pinks and other OTC stocks.

https://capitalmarkets.etrade.com/e/t/capital...seclending
Securities Lending

E*TRADE Capital Markets Securities Lending operation, with over $1.0 billion matched book and access to over $250 billion of U.S. securities, is proficient in locating hard-to-borrow securities.

Leveraging our retail trading platforms and longstanding relationships with institutions and broker-dealers worldwide, E*TRADE Capital Markets has entered into every major world market. Estimates of the size of the equity securities lending markets are in excess of US$700 billion, with significant participation from mutual funds, pension plans, endowments, and insurance companies.

E*TRADE Capital Markets has a strong foundation of capital, the ability to finance at competitive rates, and a demonstrated readiness to adapt to a changing marketplace. Clients look to E*TRADE Capital Markets to assist with their portfolio hedging strategies.

We view our securities lending clients as strategic partners and, in turn, feel it is our responsibility to provide you with a well-balanced and exceptional lending program.