Your customer's CX switches are managed in HPE Aruba Networking Central.

When a VSX primary switch is powered off, it loses connectivity with management systems such as HPE Aruba Central. This causes the management system to trigger a "Switch Disconnected" alert because the switch is no longer reachable.

"Switch Reboot" alerts would indicate a restart, not power off.

"Switch Stack Conductor Change" refers to stack leadership changes, not VSX primary down.

"Switch STP Root Change" relates to Spanning Tree Protocol root bridge changes.

Hence, powering off the VSX primary causes a "Switch Disconnected" alert.