Often, the management asks an experienced employee to impart training to newly joined employees. It may also be the case that the management may be planning to promote the employee concerned to transfer him/her to a different division and prior to that ask the employee to train other persons to take over the job.
Let us encourage each other in sharing knowledge.
Non-violence is the greatest Dharma; So too is all righteous violence
When a senior employee is asked to train a new entrant, it is often like shadowing or stepping in the shoes at similar level. The senior person would be moving on, transferred, promoted or going on a sabbatical etc. At such times, in good professional conduct, the senior has to impart what is needed for the new person to take on the role in parallel or as a replacement.
With regards to being insecure, My view is that, in the current work environment, nobody's job is safe, if one think's that he or she is irreplaceable, then it would often a mistake. No one has a unique set of skills that is not reproducible, there are always similar candidate is available.
But for sure that I will be feeling a bit down but that remains from the time we are in job till the time that we gets retired.