The cases of
Dr.disputas are completely different. An explicit hyphen makes TeX break the chunk only at it, ignoring hyphenation patterns in the rest of the word parts.
babel for Norsk or Nynorsk you can use
"- that allows hyphenation past the hyphen.
Dr.disputas is considered, the general rule for hyphenation enters: it is explained on page 254 of the TeXbook, second doubly dangerous paragraph.
TeX looks for potentially hyphenatable words by searching ahead from each glue item that is not in a math formula. The search bypasses charac- ters whose
\lccode is zero, or ligatures that begin with such characters; it also bypasses whatsits and implicit kern items, i.e., kerns that were inserted by TeX itself because of information stored with the font. If the search finds a character with nonzero
\lccode, or if it finds a ligature that begins with such a character, that character is called the starting letter. But if any other type of item occurs before a suitable starting letter is found, hyphenation is abandoned (until after the next glue item).
D is the starting letter and hyphenation is tried on
Dr, because the zero
\lccode of the period ends the trial word; of course
Dr can't be hyphened. There's no glue, so
.disputas is not good for hyphenation. Indeed,
You can use the shorthand
"" in this case: