。
The riot was caused by his instigation.
Literal
Riot [topic-は] his instigation [by-によって] occurred.
によって marks the agent/cause in a formal construction. Note that 起こった is intransitive ('occurred'), not passive — so によって here expresses cause rather than agent of a passive verb. 扇動 (instigation/agitation) is a formal compound. The sentence reads like a factual news report.