It's mainly about relativistic effects. Relativistic affects mainly affect heavy nuclei, such as polonium etc, so makes their properties slightly different to the trend.[ref] The outer electron configuration of Polonium could differ so much from its lighter family member tellurium due to the electron constriction, causing an outer electron to fall into a lower shell, that the intramolecular forces are weaker. Thus a lower boiling point.
It explains a lot of things that do not seem realistic with very heavy nuclei. Such as the "golden" colour of Gold. It should be silvery, but due to relativistic effects it is a "golden" colour. It's also like Copernicium behaving more like a noble metal, unlike the other metals of group 12, even being gaseous in room temperature.
Reference: Wikipedia article on relativistic quantum chemistry