by Why does flushing one toilet affect another toilet in the house?
Short Answer: The fixtures may be sharing a restricted drainage path or vent system.
Detailed Answer: When one flush causes bubbling, gurgling, or water movement in another toilet, the plumbing system may be experiencing airflow disruptions caused by partial blockages, vent problems, or septic restrictions.
Related Questions: Can septic failure affect multiple toilets? What causes toilet-to-toilet interaction? How do I diagnose shared drain restrictions?