Is there any chance that they may be delaying mailing them out in anticipation of an eTicketing launch, and are hoping to send out eTickets (or have people use eTickets instead of mailed tickets) for trips that are further out?
This is standard for Amtrak. The printing/mailing department isn't exactly huge. So they deal with mailing the tickets for the closest upcoming trips first, then as time permits they work into the future.
I suppose that there is also a chance that eTicketing could be having a further influence on things.