Here is an excerpt from the manual page for tcsetpgrp:

        If tcsetpgrp() is called by a member of a background process
        group in its session, and the calling process is not blocking
        or ignoring SIGTTOU, a SIGTTOU signal is sent to all members
        of this background process group.
     

The SIGTTOU signal is normally sent when a background process tries to send output to its controlling terminal. The default action is to stop the process until it receives a SIGCONT signal!