[Suicide Junkie]

I don't know how you got that impression, but there are some quirks to the operation of these weapons:

- Shields can be over maximum; when you destroy a generator via disruptor or null-space, the shield max goes down but the current strength does not.
- Shields beyond maximum will drop to maximum on the next hit.
- Ion, Tachyon, & Disruptor damage is an all-or-nothing thing: If the damage you do is less than the component's hitpoints, it does nothing.

Perhaps your disruptor was too weak (level 1 disruptor? long range attack?), but once the shields were down, the "extra damage" you did when the shields collapsed was enough to take out a generator.
Extra/Leftover damage gets added onto the next attack, and is the same damage type as the next attack. So, you probably pulled 20 damage out of the armor and added it to the shield disruptor's attack.