I think it is pure policy consideration that uscis decided to begin retrogression on Oct2005, maybe it is easier for them to start this at the beginning of the fiscal year, for some internal policy/procedural reason.
The reasoning is that it is impossible that all the visa problem suddenly appear in September 2005, that's 300k lag, obviously they already know there is a huge problem for a long time, maybe back to early 2005, but decided not to take any action (leave it C) until the begining of a new fiscal year.
Also, for this reasoning, there is no reason to believe a new fiscal year will surely begin another round of retrogression, i.e., if uscis is already in retrogression mode, there will probabily be a smooth transition.