What is one cause of the decline of the byzantine empire?

There are many causes to the decline of the Byzantine Empire.

They had many civil wars, two of which really shook up the country.
The Ottoman Turks were super strong and were expanding very rapidly, looking to Constantinople as the "City of World's Desires"
Their system for administration worked when the Empire first took root, but quickly became outdated.

The cause of its fall was pressure by the Ottoman Turks. The Ottomans had been fighting the Byzantines for over 100 years by this time. Hope this helps! ;D