Plain and simple, there are overlapping shortcuts in the Liquify filter window.
The shortcut for the Smooth Tool is the E key, however once you press the ALT key inside the Liquify window, that shortcut will no longer work, it will instead toggle the Show Mesh checkbox. Restarting the Liquify window or even Photoshop doesn't seem to help.
The other pair of overlapping shortcuts is for the U key, which will toggle the Show Guides checkbox if you haven't made any changes to the Liquify mesh, or it will press the Reconstruct button if you did.
This is in the Windows version of Photoshop CC 2015.1.1, it was also there in CC 2015.0.1