It's not leftover, it's a tie in. The movie is coming out in August, the "reboot" manga starts in May. That allows for four chapters, one volume, to be serialized, with the fourth and final chapter released the same month the movie comes out. It has already been announced that Embalming will return in the fall, though the month has not been specified. Watsuki is currently under contract with SQ, not WSJ, so it makes sense that SQ will handle the reboot. Besides, to serialize an entire volumes worth of material in WSJ he would have to write ten chapters. SQ will only require 4 - 6.
The Rurouni Kenshin movie is actually a really big deal, not a typical manga adaption. It is a big budget movie produced by Warner Brothers, an American studio. The intention is to not only have a hit in Japan but for an international theatrical run, not just airing at comic conventions and then going straight to DVD. I expect quite a few tie ins this winter and spring, not just the manga reboot.