Question: Why would liquid half-and-half creamer curdle when poured into hot coffee? This happens to me sometimes, and I'm not sure why. Could it have something to do with either the coffee being too hot or the creamer being too cold?

Answer: 
I would think this is happening because your cream is just about to turn sour. 

As cream ages, lactic acid builds up in it. The acidity in your coffee is enough at that point to push the cream over the edge to curdling. 

Try newer cream or a very low-acid coffee with old cream and you should be OK.
The acid in coffee causes the cream to curdle.