I get asked a lot about why some cats choose to do this. After doing some online research it seems that some cats just simply like drinking water this way. Like some humans prefer a straw.
Your cat may not like the shape of the water bowl, or just feel it’s easier or more convenient to drink this way. That way their whiskers won’t feel the pressure of the small cup or bowl pressing in against them. It simply is more comfortable for some cats to drink that way.