This is an application that's being installed on phones where Dropbox is marked as a system application and the only phones where this is encountered are the ones from Motorola.
I've found parts of this issue on a Reddit post, but again there was nothing to match with you issue. The only thing I can recommend is to permanently disable/freeze the app. Go to Settings > Applications > All > Dropbox and Nativedropboxagent. Tap on each one of them, then press Clear Data and Clear Cache followed by Disable. The phone will ask you if you want to return to factory defaults. Choose Yes and that's it, the app and the agent will be disabled on your device.