Exactly the same thing occurred 45 yaers ago in my old ( major) Control centre. A worker using a backhoe dig and cut a bunch of cables just outside the centre , severing all telephone and radar lines ( that were all underground at the time ) Lessons were larned to have multiple access, etc.. but one of the root cause still rermains today : sub contracting basic work to the lowest bidder. In our case the guy operating backhoe was not one the regular workers who had a briefing on the cables layout , but the digging crew that morning were hired on the day to do some basic earth removal and they were not made aware of the exact position and sensivity of the cables.
I would not be surprised if the same happenned in Frankfurt yesterday with Deuche Bahn ( the railway company that was contracting the work that cut the cable), How many sub contractors down the line ?
As to the resposnibily of Lufhansa, of course not their direct fault as the incident occured quite far away from the airport, but relying on a single source of communications to function, probably is.