Why do some Muslims fold the top right of their prayer mats?

Why do some Muslims fold the top right of their prayer mats? I asked a few who said that we should either remove the prayer mats or fold a portion of it else Shayatin will pray on it!!! IMO (atleast) Iblees is aware of Allah and perhaps Iblees is more knowledgeable about Allah than many of the Muslims alive today. The other beings who do decide to pray on those mats may actually be believing jinns. So, what is the fiqh between the folding of mats? Or is it a bid'ah?