*The official explanation is that Rodimus
ate something that didn't agree with him; the fact that said something was a product of the Nexus being on a need-to-know basis. Regardless, he's been temporarily relieved of duty while he recovers.*
*Rodimus himself is barely aware of the ramifications of his abrupt leave of absence. He's been told to lie down and rest and not to worry about anything, so that's what he's doing - with intense concentration and nary a word of protest.*