How could a fuse be triggered?

Fuses are triggered, or more correctly blown, when the current through them exceeds their preset limit. A short circuit would be one example, where you might see several hundred amperes in a 15 ampere circuit, but only for a short time, as the fuse would blow nearly instantly. An overload would be another example, where you might see 25 or 30 amperes in a 15 ampere circuit. It might take some time, seconds or minutes, depending on the overload time characteristics of the fuse because, in that case, the fuse is designed to allow short time overloads to allow motor startup.