Turning the question on its head: why is egg laying reduced or stopped,  
either in winter or prior to swarming?  
 
Speculating wildly: could it be for the same reason - workers not getting  
their full daily fix of regal pheromone?  It is 'well known' (ie open to  
challenge!) that one of the stimuli to swarming is overcrowding of a colony.  
This might be because a beekeeper hasn't supered in time; because the strain 
of  bee is unsuited to the locally practised methods; because the hollow in 
the tree  just isn't big enough any more; because a week's rain means that 
foragers have  been sitting and surviving at home rather than working and 
dying in the field  while eggs laid 3 weeks ago have been emerging at 1,000 a 
day?
 
A similar situation might happen in winter when the bees are very tightly  
clustered and movement within the colony tends to follow convection currents 
 rather than the usual pattern of interactions during which pheromones are  
transmitted.  Circumstances that reduce the tightness and ameliorate  
pheromone transmission, eg increased warmth for at least part of the day;  
reduction in numbers through natural mortality, better insulation through cells  
being empty, might ease the situation and, through resumed efficiency in  
pheromone transmission, egg laying increases again.
 
Chris

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