I'm not sure what the problem is wih these things but when you sit in a chair with wheels such as ours the mat dimples where the chair was sitting. Over time the mat will form linear cracks from the constant rolling of the chair over the surface. This is why we had to replace the last mat.
My old chair mat finally developed cracks and tears after about six (6)yeares of heavy use. When staples had the same mat on sale I immediatley bought a replacement.