The High Council elected Orion as the next Prime because Sentinel Prime was kidnapped by Decepticons. Although Orion didn't accept the title, Megatron saw this as betrayal (because he wanted to be a Prime) and declared the war against Optimus and his Autobots. (From WFC prequel comic)

After Zeta Prime has fallen during the war, Optimus had to accept the title, because the Council leader forced him (Even now, you still reject your calling?)