Why are 'sec-' and 'tert-' prefixes printed in italics?
From the ACD Labs page about IUPAC italicization,it is stated that 


  Italicizing mainly serves to mark letters which are not involved in the primary stage of alphabetical ordering.


According to the American Chemical Society Style Guide (p. 235), the use of italics for


  positional, stereochemical, configurational, and descriptive
  structural prefixes when they appear with the chemical name or formula. 


(There are several prefixes in the list).

These are separated from the chemical name with a hyphen and are not capitalised