In what way does back siphonage usually occur?

Back siphonage commonly occurs during a significant reduction in water pressure within the plumbing system. This reduction in pressure can create a vacuum effect, allowing contaminated water from a lower source to be drawn back into the potable water supply. It can happen, for example, when there is a sudden high demand for water or a break in a water main, leading to a drop in pressure.

The other situations described do not typically result in back siphonage. A surge in water demand might cause fluctuations in pressure but does not directly create the negative pressure that leads to back siphonage. Overflowing fixtures would simply spill water, often leading to different plumbing issues such as flooding, rather than drawing contaminants back. Rainwater affecting the drainage system generally pertains to flooding or drainage issues, rather than the specific mechanism of back siphonage which relies on pressure changes.