Then a stunning event occurred. I’ll let Gibbon explain.
Maximinus’ soldiers, exposed to the inclemency of the season, the contagion of disease, the horrors of famine, and the wasted land became dispirited and disaffected. Cut off from intelligence, they believed the whole empire had embraced the cause of the senate, and they were left as devoted victims to perish under the walls of Aquileia. The fierce temper of the tyrant was exasperated by disappointments which he imputed to the cowardice of his army; and his wanton and ill timed cruelty, instead of striking terror, inspired hatred, and a just desire for revenge.
A plot was formed between the Praetorian Guard and the second legion (Parthica) to murder the emperor and it was successfully carried out.