When Demon Souls released, reviews started pouring in praising the gameplay, and difficulty that it offered. Sales were enough that stores were selling out from the cult success the title was receiving. As it turns out, Atlus, the developer on Demon Souls, didn't expect it to even hit a third of the sales it ended up with.
Index holdings, parent company of Atlus, reported an estimation of 75 000 units of Demon Souls to sell. The game has sold over 280 000 units as of February 28, 2010. This dramatic increase well overshoots the original approximation and has left Atlus pleasantly surprised, for obvious reasons of course.
With sales figures this high over the original estimate, it leads to a lot of speculation of a sequel. Besides, it's not exactly rocket science. If a game sells well, it usually gets a sequel or six.
Source: Silicon Era