Board Thread:Questions and Answers/@comment-26506584-20160416022847/@comment-27441457-20160417004702

Those flowers appearing after Eren's cannonfire transformation weren't in the manga, but Isayama might've approved of the slight addition to the scene.

My guess would be it's alluding to the biological properties of Titan transformation. Maybe when Eren transformed, the surge of energy went into seeds that ended up in the ground some time before and hyperaccelerated their growth. So far the manga hasn't drawn any parallels, so it might not ever end up being explained.