Given the historical accuracy of the sources surrounding William Wallace, it seems very unlikely that we could firmly attribute contemporary riots to the death of Wallace. One of the primary sources on Wallace’s life is a poem written more than a century after his lifetime.

The film Outlaw King takes some considerable artistic licence, but I think it’s trying to at least be “plausible”.

Concerning rioting as a social phenomenon, there is considerable source material on the topic in this given period. It’s what historians termed The Crisis of the Late Medieval Period. French historian Marc Bloch was one of the first to recognize this trend in the primary sources and several generations of social historians have written supporting arguments or rebuttals to his theses.

I’m short, it’s much easier for historians to attribute riots to famine, dearth, poverty, and war in this period.