He's been making these mistakes for a while now, well, I would say Canada ish, Monza big time last year and repeatedly this year. Either he just knows Hamilton will reel him in and eventually pass him and the pressure literally gets too much and he makes a mistake or he pushes harder than he can handle to stay ahead and makes these mistakes.
I'm someone thinking he does it on purpose just so post race he can blame something besides himself. This time it's all "that has never happened to me ever in any testing, blah blah blah", it sounded rehearsed, with one of the interviews the person asked a question about the start and he said "I won't answer that, let me tell you about that mistake..." and repeated word for word the excuse he used in the press conference already. By gifting Hamilton the place he can make an excuse and do his usual "I had the win all along" thing, being beaten wheel to wheel... again, he has no excuse.
Rosberg himself and anti Hamilton fans like to forget that Rosberg has chopped off Hamilton several times in the past two years, a huge one at Canada which being directly into another corner is a big deal to chop someone off, both of Rosbergs have been onto straights with plenty of solid run off to floor it while off the track. As said, if Rosberg made better choices he would have led both the recent races this was a 'problem' for him. Had he left less room in Japan was it, and had he having lost the inside give himself room for the cutback rather than attempt to push into Hamilton he could have gotten ahead after the corner.
Rosberg just consistently picks the wrong option, Hamilton in Bahrain last time when Rosberg sent it up the inside made the right choice, backed out, swung out a bit, came into the corner at a sharper angle and got a far better exit, took the place right back. Rosberg's race craft is really poor. He can do a fast quali lap but I don't rate him as good in races at all.