I think this is an issue to do with running QGIS on a mac but I might be wrong. When I run a GDAL tool like clipper or translate GDAL seems to select the -XYZ driver regardless of what the input files are so the tool fails to run properly. If I manually edit the GDAL script to change the -XYZ to GTIFF the tool runs fine. I have been living with this for a while but it would be nice to fix it.
QGIS 2.18 kyngKhaos MAC OSX 1405.7461 El Capitan i7 32GB ram