The Scattershot catapult is not meant to do damage over time at all. Well it's awkward for me to say this, since i'm not a game director. However, the scattershot was always a support siege, primarily applying a damage debuff. You'd hit the enemy once with it and it as an aoe in a similar fashion to how subterranean assault works. Then it applies a debuff and you shoot again to refresh the debuff.
At least this is how i always saw this siege weapon. Instead of changing the damage it does i'd rather change the debuff it gives to more time on the debuff or whatever. It does make sense to have a DoT component so the debuff re-applies, not going to question that. Just feel like it's the wrong approach, that's all.