Most our our sales come from customers replacing there Rubber and Vinyl stair treads.  The reason this is happening is usually one of two causes.  The first cause is due to poor installation of the Rubber or Vinyl stair treads.   The nose of the tread has to have a tight fit to the nose of the step.  If this is not done, the stair tread will flex, and over time it will crack.  The solution for this is to make sure you have a tight fit on the replacement tread.   To make sure you get a tight fix you would want to put a 1/4″ diameter bead of 2 part epoxy in the crease of the stair tread nose, this epoxy will fill in all the gaps between the tread and the step.  The other more obvious cause is fitting a square nose stair tread on a round nose stair tread, it happens more often then you think.  So in recap, the most important fit for Rubber and Vinyl stair treads is the nose.