Many historians say it was because their Muslim regiments, numbering 140,000 in total rebelled during the height of the battle and destroyed much of their rearguard. All of a sudden they were besieged from all sides, despite this however the real end of the battle came when the King was himself captured, executed, and his head put on display which led to a total rout.
It makes sense because of the title of the thread. It is questionable whether they were the most powerful but it is not questionable that they were successful at the time.