If you remember correctly, Sev was captured once before by the Trandocians (sp) and tortured for some reason, instead of simply being put to death.
Maybe they were trying to gain information from him as to the whereabouts of his comrades, maybe they implanted a homing device somewhere in his body, or maybe they were just taking out their frustrations on him.
Whatever the case, they came back for him.
Probably to see if they could make a clone-commando of their own.
The Delta squad meets back up with Sev at some point in the near future, (due to growth acceleration and pirated advanced Camino/Trandocian technology) but itís not the real Sev. Itís a counterfeit sent to infiltrate the Republic forces.
Scorch realizes thereís something not quite right about Sev, (if thatís even possible), and actually thereís something VERY not quite right about Sev, until they realize he has no memories of anything prior to their last mission together.
Using a DNA trace they establish that the fake Sev is a copy originating from a secret Trandocian colony located somewhere near the Outer Rim Territories.
In an effort to save their fallen comrade, they disobey orders, steal a shuttle and head out to find Sev and rescue him from the evil clutches of the Trandocian Soul Pirates.
Considered deserters by their own army, with a Republic arrest warrant on their heels, and God knows what sinister fiends lay ahead, only in the next sequel will we find out if the elite Delta unit of the Republic Commandos is successful in their quest.