by Why does my shower drain emit bubbles after the toilet flushes?
Short Answer: Air displaced by restricted wastewater flow is escaping through the shower drain.
Detailed Answer: Bubbling in nearby drains often occurs when wastewater cannot move freely through the sewer or septic system. The toilet flush pushes water and air through a constrained network, and the shower drain becomes an escape point for the trapped air.
Related Questions: What causes drain bubbling? Can vent problems mimic septic failure? Why do pipes gurgle during drainage?