launch file launched node takes 100% cpu and doesn't update

edit

I'm seeing oddities with nodes taking 100% cpu and not working properly. In this example running static_transform_publisher from the command line works fine, or from a launch file that does nothing but launch that node- but here when I also launch my own demo.py node (assigned to configure_windows below) I see the static_transform_publisher process take a few seconds and to climb to 100% of a cpu core after launching.

The problem seemed to get worse when I updated from bouncy to crystal binaries on Ubuntu 18.04 a couple days ago, I saw 100% cpu occasionally in bouncy but maybe under somewhat different conditions, different combinations of running nodes from a launch script.

 def generate_launch_description():
    prefix = '/tmp/ros2/imgui_ros_demo/'
    if not os.path.exists(prefix):
        os.makedirs(prefix)

    static_tf = launch_ros.actions.Node(
            package='tf2_ros',
            node_executable='static_transform_publisher',
            node_name='static_foo',
            output='screen',
            arguments=['0.1', '0.2', '0.3', '0.4', '.5', '.6', 'map', 'foo'],
            )

    node_name = 'imgui_ros'
    params = dict(
        name = 'imgui_ros demo',
        width = 1440,
        height = 800,
        )
    param_file = prefix + node_name + '.yaml'
    with open(param_file, 'w') as outfile:
        print('opened ' + param_file + ' for yaml writing')
        data = {}
        data[node_name] = dict(ros__parameters = params)
        yaml.dump(data, outfile, default_flow_style=False)
    imgui_ros = launch_ros.actions.Node(
            package='imgui_ros', node_executable='imgui_ros_node', output='screen',
            node_name=node_name,
            # arguments=[image_manip_dir + "/data/mosaic.jpg"])
            arguments=['__params:=' + param_file],
            remappings=[])

    configure_windows = launch_ros.actions.Node(
            package='imgui_ros', node_executable='demo.py', output='screen')

    return launch.LaunchDescription([
        static_tf,
        imgui_ros,
        # If I disable configure_windows then static_transform_publisher works fine
        configure_windows,
    ])

https://github.com/lucasw/imgui_ros/b...

It's somewhat involved to get the above actually running, maybe I can make a smaller reproducible example if needed.

--- Update ---

demo.py was executing like this:

demo = Demo()
demo.run()  # make a few service calls, wait for them to complete
demo.destroy_node()
rclpy.shutdown()

I then added rclpy.spin(demo) to before the destroy_node and it looks like I'm avoiding the problem, though now I have to manually kill the nodes rather than they end have performed their service calls:

try:
    demo = Demo()
    demo.run()  # make a few service calls, wait for them to complete
    rclpy.spin(demo)
finally:
    demo.destroy_node()
    rclpy.shutdown()

But what was the problem? If one node does something bad, why would it foul up an unrelated node (neither were interacting on common topics)- except perhaps through the launch infrastructure?