As far as I know, the terminology comes from how the inheritance hierarchy is traditionally displayed, with the extending types at the bottom, and the parent types at the top.
This is a bit pointless, unless you feel like calling the following leftcasting and rightcasting.
I am not looking for opinions why the terminology is as is, but they are more than welcome as comments. I am looking for references on who first introduced up and downcasting, and why they decided on that name.