There are only a couple reasons why they would have an interim CEO at this point.
The first is that they have a merger deal in flight and the new CEO will come from that company (most likely imho).
The second reason is that MS. Mars surprised them with her resignation. I think it is unlikely that she resigned out of the blue, but it could be that she knew she was not going to be part of the operation going forward and felt that she should not be the face of the ER call under those circumstances.
I suspect they have a deal in flight but can't announce the new CEO until that is official, and that Ms. Mars did not want to stay on and host a call that was going to be all about the future (no one really cares bout the numbers for the pharmacy biz for the quarter that ended in June).
Is SHE had hosted the call, there certainly WOULD have been a question about her leaving.
A delay in a complex deal explains everything pretty well.