pushd is a bash enhancement to the POSIX-specified Bourne Shell. pushd cannot be easily implemented as a command, because the current working directory is a feature of a process that cannot be changed by child processes. (A hypothetical pushd command might do the chdir(2) call and then start a new shell, but … it wouldn’t be very usable.) pushd is a shell builtin, just like cd.
So, either change your script to start with #!/bin/bash or store the current working directory in a variable, do your work, then change back. Depends if you want a shell script that works on very reduced systems (say, a Debian build server) or if you’re fine always requiring bash.