Having spent many, many hours in promyvion farming for countless runs through I managed to work out exactally how they drop.
As people have mentioned it is dependent on the element of the particular mob that you are fighting.
Ice mobs = Somber and Malevolent
Earth mobs = Profane and Malevolent
Water mobs = Bitter and Malevolent
Dark mobs = Malevolent
Fire mobs = Burning and Radient
Thunder mobs = Startling and Radient
Wind mobs = Fleeting and Radient
Light mobs = Radient
As you can see each mob drops the memorty of its element, and of its elements type (either Light or Dark). All mobs in Promyvion can drop them except the boss from what I've seen.
As a quick aside Recollections drop as follows.
Holla : Wanderer = Fear / Weeper = Pain
Dem : Wanderer = Pain / Weeper = Guilt
Mea : Wanderer = Guilt / Weeper = Fear
Hadn't seen that posted before. If theres a guide feel free to refrence this if it helps.