Interface contracts are sets of constraints specifying valid exchanges of messages between two or more peers. A contract violation occurs when one of the peers fails to fulfil one of these constraints and emits a message that is not a valid continuation of a message "trace". In some cases, the message that directly exposes the violation turns out to be the last of a succession of forced moves, while the "root cause" of the violation resides earlier in the trace and may emanate from a different peer. We formally define the notion of causality for interface contracts expressed in a first-order extension of Linear Temporal Logic. In particular, we show how the detection of root causes reduces to satisfiability solving of a precise set of formulæ. An experimental setup shows how causality can be analyzed automatically on a pre-recorded message trace.