According to my source, the number of crew applying for the voluntary no pay leave (VNPL) for the month of October is 700+ and only a handful (less than 40) was successful. Naturally, the unsuccessful crew were disappointed. Many have planned their itinerary for the VNPL period and some even have enrolled for courses.
The sudden change in management strategy is not unusual. They are many things cabin crew management has to cope with as far as crew scheduling is concerned. One reason may be that some other airlines (Scoot/SilkAir) may have a last minute surge in the demand of seats. They may not have enough crew nor the aircrafts to accede to the demands and thus seek SIA's help.
Another reason could be that SIA suddenly mount numerous chartered flights due to strong demand from the travelling public. Hence, the inability on the part of cc management to grant all the requests for VNPL.
I am not sure whether cc management has explain to the crew the reason or reasons for not able to grant many their requests for VNPL.