that's one of those 'features' that just seem to happen with most of the rear brakes ...
there's a few things you can do to try to get rid of it, but it seems that the squeal comes back every time...
1. make some shims out of an aluminum can, pull the pads out, cut a piece of aluminum off the can and lay the pad on it as a stencil, cut the piece out and place it between the pad and the brake piston (both pads)
2. use some brake quiet goop or just some regular grey gasket maker to effectively 'glue' the piston to the back of the pad
3. just don't use the rear brake under low speed conditions... [cheeky]