Cologne - Practical Pedagogies 2018 - #2 - A little urban puzzle for you

Earlier on in the evening, there was a little urban geographical story that we encountered.
I had a wheely case, and was heading to my hotel, when we came to this space (it was dark at the time though).
It was outside the Opera house, and there were several people in fluorescent jackets who were directing people to walk around the far edge of the square, and not on the area that is shown in this picture.

The Cologne Opera House has a long history. This was the venue for Keith Jarrett's seminal Köln Concert.

Anyone have ideas on why everyone was being prevented from travelling across this square? 

I know, by the way...