Mec is pretty much physiologic in postdates babies; they don't need an anoxic event to poo.
Otherwise, mec can be caused by stress or an anoxic event that relaxes the anal sphincter. Whether that's significant or not is a matter of disagreement. Most babies with mec are fine, and deep suctioning isn't even used anymore because the theory is that mec aspiration occurs in utero in response to anoxia (baby takes a breath) and the small amount of mec in the trachea doesn't make a difference.