All Zombie process are are child processes that have exited, but their parent hasn't called wait() yet to see what the child's return code was. They don't consume any resources except a process table entry.

When the child exits/dies, the parent gets sent SIGCHLD to notify it. If it doesn't call wait() in that signal handler, the child stays a zombie until wait() is called. If you haven't defined a signal handler, the default action will be to leave it as a zombie - after all, how does the OS know that you're not going to want that return code later?

Library calls like system() do the wait() for you. popen doesn't. That's what close FH is effectively doing on a popen'ed filehandle.