Rather than the simple description of symptoms that many product users or process participants might use, the term failure cause refers to a rather complete description, including the pre-conditions under which failure occurs, how the thing was being used, proximate and ultimate/final causes (if known), and any subsidiary or resulting failures that result.
The term is part of the engineering lexicon, especially of engineers working to test and debug products or processes. Carefully observing and describing failure conditions, identifying whether failures are reproducible or transient, and hypothesizing what combination of conditions and sequence of events led to failure is part of the process of fixing design flaws or improving future iterations. The term may be applied to mechanical failure, structural failure, electrical failure and systems failure.
Over time, as more is understood about a failure, the failure cause evolves from a description of symptoms and outcomes (that is, effects) to a systematic and relatively abstract model of how, when, and why the failure comes about (that is, causes).
The more complex the product or situation, the more necessary a good understanding of its failure cause is to ensuring its proper operation (or repair). Cascading failures, for example, are particularly complex failure causes. Edge cases and corner cases are situations in which complex, unexpected, and difficult-to-debug problems often occur.