Sure, people can riot because they feel like rioting – something along the lines of life is kinda pointless but cheap thrills are fun so hey why not? – rather than because they actually want to protest something. Like when you’re walking down the street (not during a riot) and some complete stranger + nasty drunk fatally bonks you on the head and the next day can’t think of why he did it. It happens, sometimes.
The stuff you’re seeing in your country now is more about people wanting change. The fact that riotous behavior is easier in a crowd that’s already assembled means people who want to do it anyway suddenly have the opportunity when crowds happen to exist. Yet the protests that create those crowds are (basically) for the purpose of promoting certain beliefs against certain others.
Of course, people can organize protests in the hope that they will lead to riots, but that’s not nihilism either – it’s the belief in your own ability to use the beliefs of others for your benefit (as I’ve tried to explain before).
If there are more riots than usual in a given period, that probably means there are more emotionally charged protests than usual. And there have been emotionally charged protests, and riots too, for pretty much as long as humans have been capable of forming crowds (i.e. since long before Nietzsche and all that).
The way your thinking comes across to me reminds of that old cliche: son does something parents don’t like it, parents get pissed off, parents declare I have no son –
– or I have no daughter as it goes from time to time.
It seems like if you were there, you would ask the son (or daughter), why don’t you believe in family values?
(You could instead – or also – ask the same question of the parents, but that would be very un-Confucian. 
)
I would say that question misses the point. In the vast majority of cases of family conflict, it’s not yes family values vs. no family values but rather FV version A vs. FV version B. In those two literary examples I gave, Juliet justifies her disloyalty to her father by pointing out that she’s being loyal to her husband (who is also her family), and Cordelia is the only one of the three daughters whose family values have any substance – the other two put on a good show but really only care about themselves.
Actually, that last example is very resonant here, as the loyal daughter’s values are mistaken for, literally, nothing.
There’s also the occasional I have no brother/sister, like in a certain episode of Game of Thrones. And (spoiler alert) in that case, nihilism almost works as an explanation, because who wouldn’t be a nihilist after going through Ramsay Bolton’s “enhanced interrogation” / ego annihilation program? But in the end, it turns out deep down the brother never really lost his values (he was just afraid), and the sister never really stopped caring about him (she was just pissed off and/or being pragmatic). 
Um, possibly the part where you said
When people start BEHAVING as though life and existence is pointless, and not simply thinking and moaning about it…
Storming a legislature because you’re an anarchist and want to burn it down and not let it or any similar structure ever be rebuilt is one thing. Storming it to correct a perceived injustice is something else. Those people were wrong (very wrong), but there was a perfectly non-nihilistic reason for what they did.