Mine do the same - all 3 of them will stand and almost whistle for attention. In our house it seems to tie in with the time that, once the kids are down, I go to spend a bit of quiet tme with the piggies and - possibly more importantly to their little tummies - I give them their second round of fresh veg and a top up of food. I think, in our house, they're literally singing for their supper! Does your piggie chorus tie in to any familiar part of their routine?
Whatever it is... we love it!
m x