Singapore has long kept a relatively young fleet and avoids doing heavy maintenance such as 'D' checks. This is nothing new - they been doing that with their passenger fleet for decades. While it's true the newer A380s are somewhat lighter than the early build and hence more efficient (something that's pretty common on any commercial aircraft), I think the over-riding concern is that Singapore doesn't like to operate older aircraft (at least for passenger service - they tend to hang on to freighter aircraft much longer). The A380s that Singapore is returning are coming off lease - assuming they're meeting the return requirements for the aircraft, it doesn't cost them anything to give them back to the lease company.
What is new is that apparently nobody is particularly interested in obtaining Singapore used A380 aircraft.