A quick translation of what I'm reading there--at Zone Militaire (and like I said, as soon as I've talked to German friends, I'll update) :
The article, using sources close to the investigation, say colonel Georg Kelin gave false information to justify the airstrike. He was already suspected of not complying with ISAF rules to avoid these types of incidents but Colonel Klein may have assured that his troops where "in contact with the enemy" when this was not the case. The Washington Post indeed reported that the officer requested the air strike based on the information of an informant on the ground.
If and it's a big IF that turns out to be true, then well, the rest is pretty obvious.
UPDATE: I finally got some help on the German FT article. Here goes (and here I'm going with paraphrasing instead of direct translation). The article says that the Colonel claimed her had troops in contact--which he needed to justify an air strike request to the command center in Kabul. When asked what kind of contact, he said he had visual contact even though he had no soldiers anywhere near the tanker trucks. Just before the air strike, he again stated that there was immediate danger to his troops.
Now, obviously this is about rules for getting close air support. In this case, the Colonel would have to follow ISAF rules, which are very restrictive. As far I'm told, for ISAF, having troops in contact is required to get close air support but it's not supposed to be a technicality. You have to have troops being or about to be fired on. They have to be immediately threatened. All this to say, if the Colonel did lie and did break the rules, the German prosecutor in Potsdam is going to be looking into this VERY closely.