Dropping from 9 to 8 is not random, it doesn't matter how many minutes he player.
As for the center who dropped from 8 to 7 its a bit random, i have a player in the same situation (52 - 51 - 67 minutes last 3 weeks and i had 2 weeks ago a player dropping from 8 to 7 who never played more than 70 minutes or less than 50. Its unusual but not a bug.
Is not random? Maybe you want to say that it's not a bug but 5 drop with almost perfect minutes it's a LOT of random. Anyway, only two of five dropped from 9 to 8, other two dropped with DMI (one of them it was even trained so the drop could be more heavy). :)
By the way, i know that this is not a bug. I was just angry for the unluck in one of the most important weeks of the season. I posted here mostly for checking if with all the trouble with the training, there was other people that had similar problem.