I agree that it isn't a conspiracy, because it isn't any kind of secret, but the bottom line sure does play into it. Schedule/equipment changes happen in order to reduce operations expense wherever possible, but the software that reassigns seats after schedule or equipment changes doesn't do it at random.
The two big factors that affect mass seat reassignments are the FF status of the passenger and/or the price paid for the ticket. The date that you bought it doesn't come into play at all.
I learned a long time ago that actually counting on seat assignments (or tight flight schedules) is foolish because they really mean nothing; the airline can and will vacate them at the drop of a hat. I feel that all of them should just stop giving out advance seat assignments in coach; it would be more honest and save a lot of angst.