Rumor is someone hacked iCloud. There's also some talk out there that there is a forum accessible only through the DEEP WEB (DUN! DUN! DUUUNN!!!) that specializes in trading pictures hacked from celebrity phones.
Either way, the main issue here is the invasion of people's privacy. The other issue is that people absolutely do not understand that if you put something on the internet, it is accessible. Doesn't matter what kind of firewall or privacy or protection or whatever is in front of it, it CAN be accessed. It may take a lot of effort on the part of someone who is highly skilled in computer technology, but nothing on the internet is truly private.
My thing is that celebrity personalities, along with anyone in general, are fairly naive if they trust personal/sensitive information to a public cloud service. I have no sympathy for any of them, and no shame in hitting F5 in the Reddit/r/Fappening thread.
Also this was likely (as previously stated) from a 4chan pic trading circle and someone just bought in/leaked en mass. I think it's funny because many of the celebs are probably more concerned about the financial gain they missed out on (think how much people would've paid for a nude Jennifer Lawrence prior to this!) and damage to their self-brand.
Now.. c'mon, someone leak a Katy Perry album for christ's sakes...