What can cause a fixture below a slug of water in a stack to lose its trap seal?

A fixture located below a slug of water in a stack can lose its trap seal due to back-pressure. When a slug of water travels through a vertical stack, it can create a pressure variation that pushes against the water already present in the trap of the fixture beneath it. If the pressure from the slug is significant enough, it can displace or siphon out the water in the trap, resulting in a loss of the trap seal. This loss is critical because the trap seal is essential for preventing sewer gases from entering the living space.

Back-pressure scenarios such as those caused by rapid water flow in adjacent pipes or stacks lead to fluctuation in pressure that can directly affect the water levels in traps. This phenomenon highlights the importance of proper stack design and managing flow dynamics to ensure that fixtures retain their trap seals even during changing conditions in the plumbing system. Other factors like clogging or low water flow could impact the system's performance, but they don't specifically contribute to the situation of losing the trap seal in the same direct manner as back-pressure from a water slug does.