I agree with johncharles that asking them why is the only way you'll ever find out

That's the trouble, though - nobody's actually said to me "your singing's sh*t" and left me thinking "yes, but in what way?". (And if anyone did say that, I'd ignore it - at least, I hope I would!) There's not much point asking someone "why did you tell me you thought my timing was off?" (Er... because I thought your timing was off). I suppose I could say "yes, but why did you tell me you thought my timing was off?", but I can't see that getting me much further.

The question that really intrigues me is "why did a number of people independently choose one month to volunteer criticisms of a number of different aspects of my singing, never having done so before?" But I don't think anyone can necessarily answer that.

(NB Nobody has told me my timing is off. Yet.)