Actually I was told, TUE freight on Monday is held in order to keep production numbers up for Tuesday, imagine with the extra time on Monday(due to sorts being early) it would kill Tuesday numbers
That is the "practical" reason given at the operational level.
A lot of shippers were deliberately choosing either 2nd or 3rd day for their Friday shipments, knowing that they would in all likelihood be delivered on Monday (there were exceptions of course). Express knew it was going on, but didn't worry too much about it.
When they did an analysis of the "lost revenue" (the difference between what was being collected and would've been collected had the customers paid for the service which they actually received), there was a significant gap. So all freight not due on Monday was to be held for Tuesday delivery (clogging up the stations with a lot of volume they usually got out) all in an effort to get customers to pay for the service Express was providing.
The funny thing I heard, is that they are enforcing a strict gatekeeping for send-agains (to be placed in a cage as soon as Courier returns and locked up), but all this freight that is sorted on Mondays and being held for Tuesdays is just placed under the belts or collected and placed into a can that isn't "locked" (latches secured with "zip ties" to prevent potential pilferage, or some such method), not "secure" in the least.
The ramps do a decent job of splitting out the Monday freight from the Tuesday freight during the Sunday PM sorts, but there is still a lot of Tuesday commit freight that makes it through to the stations' Monday AM sorts.
Tuesday "production" is the reason given to the Couriers for not taking out Tuesday volume - the real reason (as is the real reason behind everything Express does) has solely to do with REVENUE and the attempt to get more for doing what they already are doing.
With the elimination of Express Saver, this will eliminate a lot of this issue (customers won't have a cheap alternative available for shipments going through Express).