I mean specifically when they decide to sing just "la la la la" or make some sound like that.
Can you imagine being the drummer or keyboardist in Bastille or whatever band like that and the singer comes in and he says "hey guys i've written this new song"
and he brings out his notepad and its got
[chorus]
La lal la la la

written on it. Then you're in the recording studio and you're watching the singer in your band going "la la la" and all of you are pretending you're happy with it.

Or maybe you're the singer in Bastille and you've been told "yeah mate the next song we want to be a bit 'anthemic' so we're not saying you HAVE to write a la la la chorus but something like that yeah?"

Then you have to go and sing it.

Basically do why do singers forget they're adults sometimes and start doing dumb stuff?