My little girl used to do the same thing! I even posted a video of it because I could not figure out WHY in the world she was doing that
I also bought 2 different fountains & a refillable water container. But she would "dig" those too. Luckily, she outgrew it after about a month with us. She'll still does it every now & then...but it's very rare. She did it when she was new to household. Is your youngest male new? I'm just wondering if maybe that might have something to do with the possessiveness.