If it's anything like my friend, it's because their phone has every single photo they've ever taken, it's set to do an iCloud backup, and it's over 5 GB. Very easy to do.
Or it is possible if someone gets a new phone and doesn't restore off the current backup in iCloud it will make a separate one which will go over 5gig.
To be fair most services have to force you to do something one you have ran through your free space.
If the default behavior is to keep uploading photos until shit hits the fan, then it doesn't "just work" does it? You need to be told when to start cleaning up photos from your iCloud and how to do that properly (eg without deleting photos from your phone, so you can still show them to your friends).
