I was reading the first answer to this question: Why should I use \usepackage[T1]{fontenc}?
[...] the default font encoding (OT1) of TeX is 7-bit and uses fonts that have 128 glyphs, and so do not include the accented characters as individual glyphs. So a letter ö is made by adding an accent to the existing 'o' glyph. This means, among other things, that words containing accented characters cannot be automatically hyphenated.
I can't see what is the relationship between the fact that a character like 'ö' has to be made from two different glyphs (the 'o' glyph and the accent glyph), and automatic hyphenation.