by Why does wastewater appear in the bathtub after flushing the toilet?
Short Answer: The drainage system may be backing up and forcing wastewater into lower connected fixtures.
Detailed Answer: Bathtubs often sit low in the plumbing system. When downstream flow is blocked or severely restricted, flushing a toilet can displace wastewater into the tub through shared drainage connections.
Related Questions: Is bathtub backup an emergency? What fixture backs up first during septic failure? How do I diagnose a sewer backup?