I would argue that there were two broad causes for the decline of the Ottoman Empire.
The first of these was an internal cause, namely, that the Empire was too big and too diverse to be ruled easily. The Empire was made up of so many different nationalities, many of whom wanted self-rule, that it was very difficult to keep it together.
The second type of cause was external. The Ottomans were pressured, for example, by Russian expansion and later by the British and French during WWI. This external pressure often exploited the internal splits in the Empire as well.
For these reasons, the Ottoman Empire declined and fell apart in the early 20th century.