DS has just started doing this a lot - took me a while to twig as he'd ask for a drink and it'd be gone in a few seconds and there would be juice all over the floor. (He seems to do it more when he has watered down squash). I couldn't work out how he was spilling it so quickly but it turns out he takes a mouthful and then spits it straight out!
I'm guessing this is a phase, how did you deal with it? I can't exactly withold drinks from him.