It may or may not be terror related — I think it’s too early to tell.
From the article you posted:
“An Essex Police spokeswoman said: “An incident has occurred on an aircraft. Police and partners are responding.”
I doubt they would have scrambled fighter jets, if the incident had been a drunk passenger...
Depends on if the pilots declared an emergency or not. If the pilots declared an emergency to get priority handling from ATC, the controllers may ask them to squawk (set their transponder to) 7700 just to make them easier to spot on radar and let the controllers get them a clear path to Stansted.