I hate to be cynical, but I'd be willing to bet that it's deliberately done by the celebrities themselves at least half the time.
Yes. I think most of them were on purpose. For example the twitter ones, I think you have to go through a process to tweet pics.
No (he says sarcastically) why would they do that?
well, can you think of any other reasons?
Of course. After Kate Middleton's privacy was violated for walking around topless out doors, poor prince Harry was photographed dancing nude on a table. Applying common sense, "if someone leaked photo's of a family member nude, wouldn't you be a little wary about running around naked?"
I think sometimes they are leaked, but usually not.