Just had a look in the hubpages forum. It appears that on Aug 30/31 some of them started to lose traffic.
The increase in traffic from 23rd that Indyrank noted may have just been one of G's experiments.
They can't really evaluate a domain or sub-domain without sufficient data. One way to get the data is to toss the site up, so that it gets traffic and they can monitor CTRs etc. Which would explain why in that short period Indyrank noted even sub-par pages ranking well.
Reading the forum, they only started moving the bulk of authors onto subdomains about three weeks ago. Before that it was voluntary. So that is probably creating upheaval too.
We'll need to wait a month to see if the strategy works.