Note that it is the Date constructor that is interpreting the String, not the DateFormat.format() call. The Javadoc says that when interpreting a string of decimal numbers
If the number is followed by a slash, it is regarded as a month ... unless a month has already been recognized, in which case it is regarded as a day of the month.
So the 16 is being recognised as the month. Presumably (although it doesn't say in the Javadoc), if the month is greater than 12, it subtracts 12 from it until it is less than 12 and adds 1 to the year for each subtraction.