Whilst the new rules were apparently set by the DfT, I strongly suspect that the DfT consulted on this - why else this wacky out-of-standard size - and that the easiest organisation to consult with (especially given which airport[s] all the difficulties were arising at) was the BAA.
It is well known to those of us who travel through BAA airports that it is very rare, even during peak periods, for all of the scanners at the security checkpoints to be open and staffed. This I regard as being a result of the BAA placing cost saving above customer service - even Tesco usually does its best to open more tills when there's queues these days! Good customer service would dictate that the BAA open and staff all of the scanners in peak periods and at this time of crisis - something which recent annecdotal evidence suggests that they are not doing.
Thus, based on the premise that cost saving (or at least cost control) in all likelihood has a higher priority within the BAA than customer service, the new hand baggage size rules that I strongly suspect the BAA had an input to could be interpreted as being motivated by the following:
1. Smaller size bags = less stuff inside. Less stuff = less to manually check at checkpoints. Less to check = higher throughput using a given set of resources. Higher throughput = less justification for extra staff and scanning equipment. Objective - cost saving - achieved!
2. Most carry on bags are bigger than the new rules allow. Working on the assumption that not everyone will buy a new carry on bag even if their current one exceeds the size now allowed, some travellers will be forced to check a bag that they previously would have carried on. More checked bags = fewer carried on bags. See point one above for the rest of this line of reasoning.
Andy