1. If the alternative is holding short for a long period of time waiting a contact stand to become available guaranteeing connecting passengers will miss the next flight
2. The jetties on the stand allocated have become unserviceable
3. etc

Not ideal but it happens everywhere.

I have been bussed around DOH many times and actually experienced the opposite issue to the OP which was equally irritating.
We waited 15 mins on the bus whilst the last business customer - a lady with babe in arms - attended to an issue in the lavatory, meanwhile the first of the economy buses had already left for the terminal. Clearly the QR SOP said the driver MUST wait for ALL the business class customers.