Each mob you kill in an epic quest has a small chance of dropping a scroll. Many epic quest are done by invising and running by mobs and only killing what you absolutely have to. In order to encourage people to run the quest 'correctly' the chance of an individual mob dropping a scroll should increase as you kill more mobs. Say the drop rate is 1%. It stays 1% until you kill 15 mobs then it increases to 2%. Kill 30 mobs it increase to 3%, etc. The numbers could change depending on the quest.
You can completely clear von 1 and never get a scroll. If you take the time to kill all the mobs you should be rewarded with a higher scroll drop rate.