They are supposed to be based on performance stats so most of the time those who score points for other things also get the bonus. Last two weeks they seems to have been a bit random in parts.
The EA Index has been a farce since it was introduced but it is what it is and we're all in the same boat. Personally I'd rather they did away with bonus points altogether.
but then you wouldn't get to stoof someone by 1 point after some dodgy points awardation to one of your defenders.