From reports on ARD / ZDF it seems the problem surrounds pay. Both Germanwings and Lufthansa have been targeted by the union Cockpit.
Yesterdays strike is now over, but Cockpit are not ruling out further action this week, as in their opinion, Lufthansa aren't addressing their issues. The strikes are announced at relatively short notice and generally last for half a day or so, and target sectors of the airline's business, first this week it was short haul, then long haul. Last week it was Lufthansa Cargo.
The German government and major employers like Lufthansa and Deutsche Bahn are becoming frustrated at the way in which small unions such as Cockpit and the GDL (the train drivers union) can bring disorder to the normally orderly business of trade union relations and legislation is being suggested that i believe aims to curb their powers somewhat. As the CDU/CSU are in coalition with the centre left SPD whether anything comes of such ideas is debatable.
Surely LH employees on this forum may be able to provide better / fuller information if they feel inclined to do so.