The Professor's house had a great many valuable "historical artifacts", as The Macready had been fond of saying. And when the Professor was forced to sell the house at the end of the War (something about "bad investments, poor old chap", according to Susan's father), the four Pevensie children volunteered--or were volunteered, Susan was never quite sure--to spend their Easter holiday helping him pack up the most valuable things for storage.
It was more fun than Susan had expected, because they were able to talk freely about Narnia with the Professor, and he did have a great many interesting things on shelves and in drawers and boxes all over the house. It was a bit like a treasure-hunt, if a very dusty one. But by the third morning Lucy had become a bit bored with it all, as there were too many books in foreign languages (Lucy was struggling with French at school), and not enough weapons or statues of war-goddesses from the other side of the world. So Susan was alone in one of the attics when she found a cream-colored urn, in perfect condition, with a stopper wedged most firmly in the opening.
If any of her siblings had been there, she would have noted it on her pad and packed it carefully away; but they weren't, and so she thought, "Well, it couldn't hurt to open it and peek inside, just a little."
It did, in fact, hurt. A great deal.
Therem had been a very minor Goa'uld, and not a very experienced one; it took him quite a long time to discern that Narnia was, in fact, a true experience for Susan, and not simply a dream. And he had plans for her, because he was smart enough to see that she was intelligent, relatively well-educated, and beautiful (with the proper handling).
It was not in his interest for her to spend her time playing make-believe with her siblings, when she could be learning the part he needed her to play, making connections with men in the military and politics. This world did not know the Goa'uld, and it was resistant to the kind of worship Goa'uld craved, but Therem thought that a beautiful woman might get him access to the circles of power more quickly than any of his other potential hosts would.
By the time Therem realized that Narnia was real, that there was an entire world out there that worshipped gods and not science, and where his host had been a queen--well, by then it was too late for a great many things.