Yep. Youtube, Netflix, or maybe some video or video site that does not have good compression.
Especially if the phone's settings are wrong, and Wi-Fi doesn't kick in when in range of home, or other hot-spots, so all the Internet data is going over the 3G/4G carrier 24/7...
Or conversely, the phone is acting as a Wi-Fi hotspot, and other devices are getting onto the net over 3G/4G through it.