According to various German news web sites, rumor #1 is engine failure while they wanted to set down the load on the roof (load consists of new fume outlets). According to the firefighters on site, rumor #2 is that the engine failure happened because they were low on fuel. Apparently the fire fighters wanted to pump the remaining fuel to avoid a fire, but there was no more fuel left in the tank.
Maybe they stuck the hose in the wrong hole, or maybe there was a fuel leak due to the post-"landing" damage, or maybe they are just all full of sh*t.