here is my opinion/hypothesis , NOT just based on honeybee info or data but more on my own understanding.
in the gut of the bees , there is a "primary/basal" brain as in mammals ( ex humans , therefore we have a gut feeling ).
when the gut homeostasis ( read balanced and stable and weel functioning ) is destroyed or below par , the immune system is compromised.Just as for us , then the bees work less hard/less efficiently.Because then more energy than normal is directed towards HEALING the gut.The bees then HAVE to rest in order to get better or at least WORK below par (in an engine we would say lower the RPM).
What could upset their gut?
anything that is not natural for the bees: sugar, one unique source of pollen,quick change of pollen source,virus to which they are not used because they "travelled" long distances.
Getting queens from far away is not right either.
For us humans , when we have an upset stomach , we have to slow down in order to get better.
For some other reason , the bees do not have that luxury , given the conditions we give them for most of us beekeepers.