Greyhounds and scratchings

At angle park tonight race one, number 1 is scratched but instead of the first reserve (Mr Bourkestrum) comin into that box number 10 has..
Why is this so?
Why wouldnt number 9 go in ahead unless the trainer has said no to it.. or does someone else choose which reserve they chose?
Why is this so?
Why wouldnt number 9 go in ahead unless the trainer has said no to it.. or does someone else choose which reserve they chose?