SideView is based on the DNA segments that you share with your matches. The fact that this match has been flagged as maternal doesn't mean that they have found a closer cousin on your mother's side, only that the DNA you share with this cousin seems to be from your mother's side.I have found one today has been given an exclamation mark by Ancestry. They have worked out my match to be maternal. But the cousin is at the end of a branch in my tree on the paternal side. To be flagged as maternal they must have found a closer cousin on my mum's side but I can't work out how. And classification should probably be 'both'
Remember that someone who is more distant than a 2nd cousin might share no DNA at all with you, whereas someone who is an 8th cousin could share 20cM or more.