The sequence of events follows the "Physical Layer $\rightarrow$ BFD Protocol $\rightarrow$ Upper-Layer Application" causality chain:

1. Link fault alarm: When the physical link is disconnected, the physical layer interface immediately detects the loss of signal and reports a physical link fault (Link Down). This is the root trigger.
2. The BFD session goes down.: BFD is linked to the interface status. Once the physical interface reports a fault (or BFD stops receiving packets), the BFD local session state transitions to Down.
3. Interrupted OSPF neighbor relationship: Upon transitioning to the Down state, the BFD process notifies the upper-layer application (OSPF). OSPF reacts to this notification by immediately tearing down the neighbor adjacency, without waiting for the standard Hello/Dead timers to expire.

Huawei Technologies Co., Ltd., "BFD Configuration Guide - BFD for OSPF," Huawei VRP Documentation. This document details the interaction flow: "The BFD session quickly detects a link fault and notifies OSPF... OSPF then quickly responds to the network topology change."

Katz, D., & Ward, D. (2010). Bidirectional Forwarding Detection (BFD). IETF RFC 5880. Section 6.8.6. (Describes the transition to the Down state upon detection of a failure).

Huawei Support, "Overview of BFD," Huawei Enterprise Support Community. (Describes the "Hardware detection" mechanism where link faults are detected and propagated to BFD).