Third party sales for the Wii will probably increase at a pretty steady rate for a wide variety of reasons, and some people will attribute this new-found success to other reasons which are about as likely as a voodoo curse.
Quality: Because of the massive sales of the Wii, the Wii has become far more of a focus of third party publishers and is seeing far better games from them in general. In the first half of the year you (typically) get around 1/3 of the good releases of a year, and so far the Wii has seen (roughly) 2/3 as many titles of good third party titles this year as it saw all of last year.
Marketing: As a side effect of putting more money and effort into their games, third party publishers will be spending more money on marketing in order to ensure that their games are as profitable as they can be.
Numbers: In a lot of way third parties will gain sales for the same reason shovelware games sell. A lot of games are bought with little forethought or research by customers, and they will go into a store and look at a rack of games and buy one almost at random. As a system's userbase passes that 50 Million mark there will be (as a rough guestimate) 1 Million of these game sales in any given month, and third party games that are based on well known IPs stand a really good chance of gaining a massive portion of these sales.
