The person who the drink is being poured for is saying that it's too much and it's about to spill. The person pouring the drink wants to continue pouring out of generosity. "o-to-to" (sort of like "oops") and "maa maa" ("no no" / "doesn't matter") are normal things for the respective parties to say in this situation. It just won't go on for that long, or be done every time with those exact words like a ritual, which is the joke.
