How common? Not very as this isn't done very often. I was staggered when I found out where these were going. That's am incredibly long way in seas which are not mill ponds. The size of the tow cable looks puny given these sections appear to be floated and not submerged or barely afloat
Unfortunately there isn't much other info to see why they broke away from the tug.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.