When the message is generated from NDL999 the trace is automatically opened.
This event has already been reported by other customers.
Several questions/cases are regarding a fix to this problem.

This Buffer Trace is a expect behavour and is used to assist debugging the NDL999 error.
Feature was added in 7.3.2.
The buffer trace can be mapped to MAILBOX ENTRY DESCRIPTION in the Systems Programmer Reference Guide.
It is not to be fixed and ouput messages to help in debugging when needed