I'm wondering if it's the mms amongst themselves to launder funds from other transactions.

Say XXXX needs to transfer $1000 in cash (keeping it simple) to ZZZZ, so they do so by XXXX "buying" shares from ZZZZ and then ZZZZ immediately buys them back for less at the bid, and in the process leaves $1,000 "profit" for ZZZZ to pay a debt (money transferred owed from another reason, trade on other stock, interest, WHATEVER) paid off.

And you wonder why your orders don't get filled?

Maybe that's one reason.