The input filter capacitors in your Li-Po charger are empty when it's not connected to anything. When you connect them, the empty capacitor needs to be filled, the PSU sees this "filling up" process as a short circuit momentarily. However, PSU's usually have an output capacitor, so your basically hooking one charged capacitor to another empty one. Most all of the PSU's that I have seen won't care about this current wise because they don't have current sense after the output filter capacitor. So I would not suspect the over current to get tripped from something like that.
I do however see a lot of computer PSU's with comparator sensed under/over voltage shutoffs that are attached to these output caps. So it's more likely that the under voltage is getting tripped when the caps charge sags. In order to be tripping over current protection, your chargers capacitors would have to first drain away the PSU's output capacitor, then draw enough current from the inductor for a large enough current to be seen up by the over current sense. Server PSU's are completely different animals though, as well as the so called "multi-rail" PSU's. Both these could easily put current sense as the first thing right after the wires.
Also note that those sparks will not happen when you attach our load circuit because it does not have an input capacitance like your chargers do.
I think what ronv is talking about is the fact that when you remove the leads to the load while it's running the high current going through the stray inductance of the rest of the circuit (cables mostly) will have no where to go and will thus cause a high voltage spike from "
inductive kick". And as he said "The transorb or transient supressor may protect it." Or we could devise a
snubber circuit that should work well for such things.