do_execve() finding vmspace_destroyed set under load

I'm seeing the following code in do_execve() frequently being
triggered when scripts are executed on my SMP machine under load:

        if (imgp->vmspace_destroyed) {
                 /* sorry, no more process anymore. exit gracefully */
#ifdef MAC
                mac_execve_exit(imgp);
                if (interplabel != NULL)
                        mac_vnode_label_free(interplabel);
#endif
                exit1(td, W_EXITCODE(0, SIGABRT));
                /* NOT REACHED */
                error = 0;
        }

Needless to say, the scripts get pretty unhappy when they're summarily
aborted.  What is the cause of this?

Kris