Greasy ground while mounted

Even when mounted, and moving across a slippery surface like grease (and probably ice too), both me and the mount can slip on the surface. How does a rider slip on a slick surface when his feet aren't on the ground? A Bend in an Extinct Lava Tube The ground is slick and greasy here. [Exits: north east] A skeletal apparition hovers in midair. The ground in this area is slick with yellowed grease. You slip and fall! Your sudden fall grazes you. A vicious warg has arrived. A vicious warg slips on the grease and hits the ground hard! A vicious warg's sudden fall devastates it! <97%hp/97%hp 88%m/88%m 100%mv/100%mv> 4 PM 9172tnl (4.46%) wilderness e An Extinct Lava Tube The ground is slick and greasy here. [Exits: east west] The ground in this area is slick with yellowed grease. You manage to maintain your footing on the slippery grease. A vicious warg has arrived. <97%hp/97%hp 88%m/88%m 100%mv/100%mv> 4 PM 9172tnl (4.46%) wilderness mount You are already mounted.