Does rospy.init_node need to be called everytime?

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I developed two ROS packages and, until very recently, 1-2 days ago, I was calling one script from package A (that I developed) with rosrun (without ever calling roscore or whatever), which uses another package that I developed B. None of these packages, i.e. A or B, call rospy.init_node. However, today, I tried again to run package A's with rosrun A the_script (as usual), and I get the error

  line 150, in sync
    filters[topics.index(topic)].signalMessage(msg)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/message_filters/__init__.py", line 58, in signalMessage
    cb(*(msg + args))
  File "/opt/ros/melodic/lib/python2.7/dist-packages/message_filters/__init__.py", line 264, in add
    now = rospy.Time.now()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/rostime.py", line 155, in now
    return get_rostime()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/rostime.py", line 190, in get_rostime
    raise rospy.exceptions.ROSInitException("time is not initialized. Have you called init_node()?")
rospy.exceptions.ROSInitException: time is not initialized. Have you called init_node()?

I am using signalMessage to synchronize a .bag file and the problem is caused by this, given that the error is raised at line filters[topics.index(topic)].signalMessage(msg).

But, as I said above, I never called init_node before and I never got this error and now I am getting it, for some reason. Why? How do I solve this problem? I didn't update ROS (as far as I know), but I only executed the latest software updates in Ubuntu. In fact, cat /var/log/dpkg.log | grep "\ install\ " does not apparently produce any ROS-related package. By the way, I already tried to do catkin build, but I still get the error.

Well, I added rospy.init_node('bag_synchronizer', anonymous=True, disable_signals=True) to the class that uses signalMessage, then I roscore and executed the script again, and now I don't have the error anymore, but I would like to know why I was able to execute that script without rospy.init_node before (I did this so many times) and now I can't do it anymore. My code does not see to publish or read topics, so I don't need to initialised the node, as far I understand.