Sometimes brakes can squeak if they don't have anti squeak shims or if the pads or rotors become glazed because of heat that can cause the brakes to squeak.
But if the brakes have never squeaked before and are now suddenly squeaking then you should have the brakes checked out because it could mean that the brake pads or brake shoes are wearing down and need replaced.
Some brake pads and brake shoes do begin to squeak as they wear down overtime.