To use skills in succession means to queue two (or more) actions (skills on the skill bar or actions) by invoking one then the other (while the first one is still activating in the case of skill). Skills are not in succession if there is any delay between activating skills (even a fraction of a second) where the character stands upright again.
For example, if a player casts Fireball or Rodgort's Invocation on computer-controlled enemies, they will generally not flee. When cast in succession, the same two spells causes enemies to flee (as if under AoE attack). Waiting a fraction of a second before queuing the second skill prevents the monsters from fleeing.
This applies similar skills such as Zealot's Fire and Mark of Pain, which does AoE damage when a condition is met. Monsters flee when that condition is triggered by actions performed in succession. Examples include casting appropriate spells in succession while enchanted with Zealot's Fire, or attacking twice in succession on a monster hexed with Mark of Pain.
Note that some skills do not trigger the AoE scatter response. The most noteable of these is Spiteful Spirit, which can deal large amounts of damage to a group of enemies in a short time span and not cause the monsters to scatter.