Falling out of RANGE and falling out of TUNE are two very different things.
Dave has fallen in and out of TUNE his entire life. And that's OK because live performances are not supposed to be pitch perfect.
As for RANGE....you don't truly "fall" in or out of it. Range is a conscious choice by the singer based on how their voice is conditioned and "where" it can sing at. As singers get older they usually have perform in a different range to accommodate the changes in their voice.
Dave has apparently not reached that level of acceptance. Does he have some idea of what he's doing? Probably, yes. The problem is that he doesn't give a fuck about it.