It’s not just for hijack, also for any threats made against the aircraft and related issues.
The Air India and the Jet2 flights over the last month both came in due to threats.
Stansted has a contingency plan and operation for these type of instances.
I was at work when the Air India flight came in and the only normal operation was only affected by outbounds waiting to push or hold on the taxi and inbounds holding in the air. This only went on for 15 minutes.