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.
I watched an episode of Blue Bloods where Mr. Selleck explained to his grandkids that the secret to not having your cream curdle in your (Irish) coffee was to hold your spoon upside down over your coffee and pour the cream over the back of the spoon. I tried it and have never had curdled cream in my coffee since. Don't know why it works I just know that it does. I'm guessing it might have something to do with increasing the dispersion of the cream.