including Cyberball 2072, Defender, Gauntlet, Joust, Paperboy, Robotron 2084, Rootbeer Tapper, Smash TV and Ultimate Mortal Kombat 3. The titles have been pulled due to "publisher evolving rights and permissions," according to a Major Nelson tweet, but will remain playable and can be re-downloaded by users that previously purchased any of the games. While Major Nelson didn't expressly say so, it would appear that the disappearing titles are the result of Warner Bros.' acquisition of Midway last year. Warner Bros. now owns at least some of the trademarks involved in the removal from XBLA.
You're not alone in suspecting that some of the affected titles were pulled in anticipation of Microsoft's upcoming Game Room (where different versions of the same games could presumably be made available for purchase). After all, the simply emulated Game Room games don't stack up well against their more fully-featured XBLA counterparts. However, Major Nelson has quashed that theory, tweeting, "No, it's not Game room related."
This is not the first time that XBLA content has been pulled because of changing publisher relationships. Lost Cities was removed from XBLA last year after licensing rights were lost in the Activision-Vivendi merger.