Don't get me wrong, I think it was a bad call, but 2 things doomed him:
#1- he did step forward and upward just as he made contact, which was not a good move, and
#2- the QB tripped over his own lineman and fell
I think if the QB hadn't fallen down, there might not even be a flag. Since he did fall down, it called attention to Takitaki's step up into him.