Why is Pexpect intermittently hanging (not detecting EOF) after executing certain commands?

Context:

I have some code written using pexpect, whose job is to give "live" output of a command. I.e. print something out when a command generates some output, or soon after, rather than waiting until the command completes and then interacting with its output.

All I'm doing is starting and stopping a service. I do this by spawning the process, and then outputting each line as it is printed, like so:

def watch(process):
    output = ""
    while True:
        try:
            line = process.read_nonblocking(timeout = -1)
            print(line, end ="")
            output += line
        except pexpect.EOF:
            break
    del process
    return output

while True:
    print("IN 1")
    process = pexpect.spawn("service",["zend-server", "stop"], timeout = None)
    watch(process)
    print("OUT 1")

    print("IN 2")
    process = pexpect.spawn("service",["zend-server", "start"], timeout = None)
    watch(process)
    print("OUT 2")

This code should just loop the service: start it and stop it over and over, printing the output of the start/stop as it goes. It prints the output fine. However, it eventually hangs right before "OUT 2". I can view the output, and see the service call stop its execution. The watch function never raises an EOF and exits, however.

This doesn't happen with every service. Some services loop indefinitely. zend-server, however, along with a few other unrelated commands, fail intermittently in the same way.

By "eventually hangs", I mean that it starts/stops the service a few (variable on each run) times, and hangs. It usually gums up after 4-6, though never on the first call--always at least on the second (hence the del statement; I figured I'd play it safe).

Python 2.6.6, CentOS (64) 6.3, Pexpect 2.3-6, FWIW

Question:

Why is pe