"A Challenger Appears" is a catchphrase derived from fighting games. It is commonly used to signify the presence of something superior to the topic at hand. For example, if Anonymous starts a thread about Longcat, it is almost certain that Tacgnol will be mentioned sometime in that thread with the phrase being used.
A description of the meme is given in Urban Dictionary but there is no evidence to suggest that Super Smash Bros. Brawl was the reason for the re-popularization of the phrase OTI other than it being under the "Super Smash Bros" heading on Macrochan.
"A Challenger Appears" first appeared in Street Fighter II and has reappeared in several other fighting games after it. This phrase (or anything similar to it) is stated when a new character or a second player comes into play to fight the original player after or during a fight.
The catch phrase had somehow found its way onto the internet and had become a meme since its reintroduction some time during late 2006 for reasons (as of now) unknown.