You may be wondering why i'm asking this but when i tell you you'll understand.
1. Crazy Dave speaks his 'own' language. But what if this isn't a lanuage but groans of a turning zombie?
1.5. When Dave says "BECAUSE I'M CRAAAAAZY!" it may be a warning to the player that Dave knows that he's turning but the player may think that this is just another mumbling out of Dave.
2. When you first incounter Dr. Zomboss in Egypt he refers to Crazy Dave as David, meaning that Dr. Zomboss may have been close to Dave before.
2.5. When you incounter Dr. Zomboss yet again in the Far Future he says about Dave being a sleeper agent and 'will turn against the player upon saying "Hypno-Zombnosis." When the fight between the Zombot Tomarrow-tron is over Crazy Dave will ask for the Player's brain but quickly 'corrects' himself and say's that he ment Hot Sause.