So why does this happen? Most windows older than 15 years are at a higher risk of fogging due to their age. I've also seen windows less than 2 years old which had fogging, there are several reasons why, here are some of those:
1. Poor installation the seals were damaged in the process of installing the window 2. Low quality windows 3. Manufacturing error, coming bad from the maker of the window. 4. A small crack in the window itself.