Firing too hot for the shape of the mould is apparently what you did. The fact that the glass is thicker now than before slumping shows that you reached a temperature that allowed the glass to flow. You do not give any information on your firing schedule or the shape of the mould. Both affect the result.
You should be watching in brief glances the progress of the slump. When it is complete is the maximum temperature you should be using for that mould and that thickness of glass. If you reduce the temperature from that by 20C or 30C and soak for longer, you will have fewer marks on the bottom of the glass.
There are a number of tips here that might help you understand the result you got.