First, the turn of the century (19th to 20th) did not automatically separate theatre history. A better division might be before and after WWI, since theatre growth continued well into the 20th century. The Abbey Theatre riots, for example, occurred in 1907, when Synge used the word “shift” in Playboy of the Western World. In the U.S., the Astor Riot was in 1849. While this riot took place in the theatre district, it was not about the theatre iself, but about rival immigrant gangs and the police. The main point is that theatres were simply the biggest gathering places of crowds – the riots could be sparked by any incident, or none, at the point where unruly crowds found energy from the large numbers of unhappy, or drunken, or unemployed persons (mainly young men), and a too-small police force to keep them in line.
The parallel today might be the Arab Spring, which was ignited by political unrest, but gathered its fuel via the social networks on the Internet. In the U.S., something of the same dynamic was at work in the so-called Wall Street movement.