"What could be the probable cause???."

With nothing offered as to the operating parameters of the system the probable cause might be an inexperienced operator. What is your title?

Do you have an engineer on staff?

Have you consulted the trouble shooting section of the operators manual.

Have you consulted the manufacturer?

Are you qualified to work on high voltage switch gear? No? Didn't think so.

The probable cause is a fundamental lack of understanding of basic protective relaying practice, particularly in how to properly coordinate the Local Breaker Backup (LBB) relay settings. I suggest that you read this document...

http://www.pes-psrc.org/k/k2/PC%2037119%20draf7%2005-16-05.pdf

...and the related material so that you will be conversant with the Consulting Engineer that should your job for you.

LBB protection needs the feeder protection / feeder lockout relay operated contact to start. Once the said contact is available, the relay waits for 200ms (typical setting) for the subject feeder breaker to trip. If that doesn't happen (identified by the continued current flow in the feeder or CB close status), the trip is sent to upstream breakers, i.e. transformer incomer breaker & bus coupler breaker in your case.

So, LBB trip at the incomer without any outgoing feeder fault implies scheme problem or wiring mistake. Not an easy task to detect and rectify.

All the best.