SoC and all AOE's have a damage cap.
13,590 is the SoC cap for lock with 1000 +damage, no shadow mastery, no demonic sacrifice and not any curses, shadow weaving etc. Aka cap of the base damage. The %damage increase and the crits are calulated after it.

The cap is the damage that the AoE spell deals to single target with your current gear, multiplied by 9.

For SoC is ( 1100-1290 + 22% of your spell damage ) * your multipliers (aka DS). This is multiplied by 9. After it is divided by the number of mobs and the value is applied to each mob. After it can crit.

With less words - if u AoE 100 mobs, the total damage done to them is the total damage equal of AoEing 9 mobs.

For SoC, without Shadow Mastery and without DS. With 1000 +damage the cap is around 15K. Without Shadow Mastery or Demonic Sacrifice is around 13.5k