Explanation: The Roman Emperor Gallenius was abruptly woken up and lured out of his tent by being told that the city he was besieging was sallying forth for an attack - and then promptly getting stabbed by an assassin, one of his own officers. Big RIP!
Oh, it was just the usual situation of "I can do better" + "His support is waning right now and I REALLY want to be Emperor". Dime a dozen, too many emperors to count lost to that simple ambition. Gallienus had been struggling with several major revolts for a long period of time, which probably fueled frustrations against him.