One of the choirs I sing in used to have problems with intonation – in particular, it would lose pitch when unaccompanied. Rehearsing with them recently, I realised that this was much less of a problem now. What has happened? Are we all better at listening to one another and correcting pitch? Have some people who used to drag the pitch down now left? Or has some new blood come in recently and improved matters?
I’m not going to invite speculation about this particular group of singers, but I have one observation to make: if the pitch is really sagging in one part (especially if the people concerned don’t realise it), the other parts can’t easily pull it up again. At worst, you can get an effect of bitonality as (say) the sopranos go sharp while some of the lower voices slide in the other direction.