In Bill and Ted's Bogus Journey, Rufus presumes that his old teacher, De Nomolos is dead....Why?
Although it's not mentioned canonically, I think it's most likely that he faked his own death so that he could work on his plan to change the past by stealing a time booth and building his (presumably illegal) evil Bill and Ted robots without attracting the attention of the authorities.
Per the BBC review of the film
"2691, San Dimas, California. Three years on, and the future might not be looking so great for Bill and Ted. Chuck De Nomolos (Joss Ackland), presumed dead, isn't happy about his present being built upon the legacy of two fools and he wants to change all that. In the future, our Rufus (George Carlin) is teaching the 'physics of acoustical reverberation' at the Bill and Ted University with special guest speakers thanks to the time travelling phone booth."