Dropbox scans user accounts. It alerts the police that objectionable material appears on an account.
The police attempt to associate the account with a person. Social media profiles and IP adresses turn up a name and general geographic location.
Police find a person's name associated with said profiles, email, and IP.
They show up at his address, and ask him, hey, are these accounts yours?
"Maybe, why?"
The police then execute a warrant, confiscate his phone.
The phone contains an app, which is logged into an account with objectionable material on it.
You are now charged with possession of child porn.
Child porn is a terrible and heinous crime. Put that aside for a moment.
What happens if someone illegally accessed your account and placed those images there?