Why are 32.786Khz crystal cans soldered to PCB?
"Radiated energy" is the right idea, but your question suggests that you are thinking about electromagnetic energy here.  It's rather mechanical.  32768Hz is ultrasound with an arial wavelength of about 1cm, and a crystal oscillator works via mechanical vibration.  If you want to have constant Q and constant frequency response, fixing the case mechanically makes sense.