[Oddey]

The more I think about it, the more I think that Sekto called up Raider because he secretly suspected Stranger had a connection to Steef, say some knowledge about where they might still be around. He wanted Raider to intercept Stranger and force out whatever Stranger knew, much like what happened, presumably so Sekto could hunt them there himself. Only thing he wasn't counting on was that Stranger would be a Steef himself and get wind of Sekto being after him.

As for why it was Raider specifically, I'd say it was because Sekto wasn't about to do it himself and his Gloktigi wouldn't be suited for the task. I assume he chose Raider over his Wolvarks because Raider was a little stronger and a little smarter than any Wolvark seems to be and had a band of outlaws that were with him no matter what; that kind of ranking or leadership doesn't seem particularly present with Wolvarks, at least not from what we see in the game.