Whatever it is, it's logarithmic.
It happens on both the bid and the ask (usually on the bid), and it usually happens a few seconds after a 'normal' trade occurs on the opposite side.
Imagine having an open GTC order valued at $100. You're first in line, and get hit with a 100 share fill a day for 10 days in a row, * $9.99 commission.
I agree, FINRA might know.