How stupid do you have to be? (Motorcyclists slow down canned drivers)

Moin, the days on a two-lane road near Frankfurt – I drive right at about 120 during the day, run on slower vehicles, which drive about 100. Mirror view, motorcycle far behind on the left lane, waiting a few seconds, looking again, motorcycle still far away (I saw more from the headlight than the actual vehicle). So turn signals set, left over and the overtaking started. After maybe 15 seconds the mopped behind me, closes up – and d overtakes me left, not half a meter away from the guardrail. If then sit in front of me (mark was bent up in the middle, well clear) and goes about four car lengths away completely into the iron. I put a full brake down and drove up to about 5 m on it, then it turns up again and runs off. Identification was of course not in and you can do there now also nothing more. Obviously I was in his way, although I did not endanger him when changing track – he was as described still far away. Explain to me – a motorcyclist has absolutely bad cards from behind in a car crash, right? He could have been thrown into traffic on the right lane. What has ridden the guy, how can you put yourself in such danger?