Any number of reasons.
First, as Podrock suggests, gravity is a variable. It is stronger some days than others.
Second, a big bird landing on the edge, or a cat jumping on it.
Third, a typical 6 year old who thinks that ceramic bowls with rocks in them are for tipping over.
Fourth, most likely, you were being chased home by a bunch of guys from the local bistro and your purse brushed against it.