The newRoot trap indicates that the sending agent has become the new root of the Spanning Tree; the trap is sent by a bridge soon after its election as the new root, e.g., upon expiration of the Topology Change Timer, immediately subsequent to its election. Implementation of this trap is optional.
Sources for this description
Zyxel official Download Library MIBsZyxel MIB copies (LibreNMS/Observium Community)