It might be the user's upstream bandwidth from their computer to the BigBlueButton server. If it is too low, at some point, the BigBlueButton client or BigBlueButton server has closed the video stream.
We've added logic in BigBlueButton 1.0.0-dev to automatically reconnect the video stream if the client detects it has closed.
Is this reproducible with your server from a given computer? If so, could you connect to
and share your video and see if (a) it drops and (b) if the BigBlueButton client on demo handles the automatic reconnect.
Regards,... Fred