There are a variety of factors that could contribute to rapid data usage.
The main way to use a lot of data quickly is by downloading or streaming audio/video (e.g. Spotify, Netflix, P2P); this can easily result in data usage of over 100MB in 15 minutes. For your reference, a standard-definition movie is usually around 700MB.
In addition, certain programs will consume data in the background without giving any indication of doing so. For example, system updates are usually downloaded and installed automatically without prompt. Backup software (such as iCloud, Dropbox and GoogleDrive) will continuously use data throughout the day - in fact most Mac / iOS devices come pre-configured with a backup feature always enabled. Other programs such as Skype and other chat clients will also contribute to data usage if kept on continuously.
Finally, you should be wary of email clients such as Outlook and Entourage, which constantly download both emails and attachments.
Remember that any data usage is amplified when sharing the connection between multiple devices or people, meaning even 'normal' activities such as web browsing will consume more data when multiple devices are connected at the same time. For that reason, we recommend you share the above information with anyone who will be using the Portable Hotspot with you!