I have been doing security at theatres for about 20 years on and off. It was mainly introduced as a presence and bag checking after 9/11 and ramped up after 7/7, but quickly evolved into dealing with the poor behaviour of drunk guests.
People really dont know how to behave at the theatre and it has got steadily worse over the years. People turn up with take away food and pizza boxes to eat when the show is on, sing and dance when they like and get abusive when get asked to behave, and then try and fight when we try to remove them. Unfortunately the venues I have worked at rarely call the Police as they do not want the adverse publicity but good on this theatre for doing so.
It is always clear when performances are sing a long or not, or if photos are allowed to be taken or not, but in recent years people have just become so much more entitled and do what they like.
IMHO theatre managers often don't like dealing with the complaints from people who act up, amd just it to security to deal with. I have seen people ejected for genuine reasons and then theatre managers give them free tickets to future shows to avoid bad reviews.
There is not easy solution I am afraid, it seems to me to be a general downward decline in the way a more visible minority feel entitled to do what they like and bollocks to anyone else.