Someone sent a message to Stephanie, instructing her to intercept the goods in the Rocklands. I suppose it must have been the people from the Bane Church faction who were behind this.
The information provided by Shanoba is of very low quality; she probably isn't even clear on the significance of names like Manson and Simmon within the Santarin Association.
But Gaven, combining his knowledge from his past life, quickly pieced the entire situation together.
Manson is the founder of the Santarin Association and the undisputed master of the Dark Intelligence Net—at least for now. Although he hasn't fully sided with the Shylock Faction, he has tacitly allowed their ascent.
Simmon is Manson's top henchman, a former Apprentice, a powerful Mage approaching the status of Legendary, dispatched to Dark Castle to lay the groundwork for new trade routes.
If this batch of goods was indeed supported by Manson for Simmon,
then the person standing behind Stephanie must be extraordinary.