CAl/ASU Fumble and Review

[jimpiano]

So the Cal player is under the punt and as he catches the ball and is falling down he falls on an ASU defender and fumbles the ball which is recovered by ASU.

But it is clear that the Back Judge is stopping the play before the fumble.

After a conference by the officials the ruling on the field is fumble and ASU's ball.

The play is then reviewed .

After 3 minutes of review the ruling is that the play was dead by the
"inadvertent signal" by the Back Judge and the ball belongs to Cal.

(whether the ball was fumbled or the play was over by contact with the ground,which is now immaterial, is not clear on the replay)

So the questions are:
Why was there a review?
If the back judge stopped the play why would there be a ruling on the field of fumble?