That, and it got a bump when it got involved in Bat crossovers.
Issue #8 moved an estimated 43,990 units, then issue #9 moved an estimated 58,710 units. Simone's run was definitely doing a lot better, even without help from crossovers/Bat events and the like, but there's more going on with the drop in sales than the change in direction, because the Burnside direction saw an increase in sales at first, with issue #35 moving 62,644 copies, and #36 moving 50,247 copies, plus the book charting better digitally.
It was heading towards stabilizing in the mid 30, 000 range, but it just fizzled out after the Convergence break.
Did people just get tired of the direction? Did new readers lose interest after going two months without a new issue from the book?
The Rebirth book is doing about 5000 copies less than the Stephanie Brown run, and is probably performing the same when you take digital sales into account, but as evidenced by both the Simone run and the Burnside run, Batgirl can sell quite a bit better than it is now.