Why did water fill up kitchen sink and flood when not in use?

At least three possible answers.

The sink has the plug in the drain or the drain is blocked in some fashion and --

1. The hot tap leaks or has not been turned off correctly,

2. The cold tap leaks or has not been turned off correctly,

3. The temperature difference between the material of which the basin is constructed and the surrounds is such that condensation forms inside the basin,

but then a couple more occurred to me ---

4. Some other person puts the water in it - to annoy you,

5. An animal which expels water puts the water in it (i.e. pees in it)

and,

I'm sure someone else can think of more answers. . . .and you didn't say whether it is clean or contaminated water.