Why do intercepts fail so often?

[stoplion]

I've set up a intercepts to intercept API endpoints when testing. Those intercepts return a JSON fixture.
Problem is ½ the time the intercepts fail themselves. Why do intercepts fail?
Seems like the only job of an intercept is to listen to outbound requests that match a URL pattern and return a fixture. Why do they fail so frequently?

[stoplion]

I should add I am aliasing the intercept and waiting for it to complete. In the waiting period, it fails very often.