Why did Avidius Cassius proclaim himself emperor in 175 CE?

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Bronze equestrian statue of Marcus Aurelius with his right arm raised, seated on a horse, at Piazza del Campidoglio in Rome, Italy. — Marcus Aurelius Marcus Aurelius, bronze equestrian statue in the Piazza del Campidoglio, Rome.

In 175 CE, the Roman general Gaius Avidius Cassius, who had become the supreme commander of the eastern provinces, proclaimed himself emperor after receiving a false report that Marcus Aurelius had died while campaigning on the Danube frontier.12

At the time of this event, Avidius Cassius held significant authority over the eastern territories, including Egypt, and had previously distinguished himself as a capable military leader during Rome’s war against the Parthians. Upon hearing the rumor of the emperor's death, he took the opportunity to seize power. Marcus Aurelius responded by preparing to march against the usurper, but the conflict ended abruptly when Avidius Cassius was assassinated by his own soldiers.12