Whether robbing takes place or not in an apiary is down to the Waggle Dance, which forager-scouts use to (somehow) communicate with other bees the geographical coordinates of a food source - this applies whether the food source is 'out there in the fields' or lies within a neighbouring hive.
If, during a flow, a particular forager-scout detects a source of food within a newly-made nuc or a weak colony, that scout's dance will be drowned-out within it's own (or parent) hive by the numerous dances of other scouts who have detected food 'out there' in the fields - but - during a dearth of nectar, that scout's dance will then be the only dance in town, and so other forager bees will listen to it and respond accordingly. At first a few scouts will fly out to check for validity and ease of access, and if upon their return they add to the same information, then at some point a threshold level is reached, at which point classic full-scale mob-handed robbing begins.
However, there is another type of robbing you should be aware of, which can occur within a single-yard apiary. This is sometimes called 'passive robbing', but I prefer the term 'latent robbing' for it is effectively hidden from the beekeeper, who interprets the entrance activity as nothing more than normal hive traffic. It occurs thus:
When a nuc is made-up, (say, from a split) then it is almost guaranteed that a handful of bees within that nuc will be forager-class bees. Over the hours and days that follow, those bees will fly back to the hive from which the nuc was made. If there is a dearth of nectar in progress, then those foragers are in possession of the food source coordinates of the nuc box, and so can readily return to it. Upon arriving at the nuc box, they are able to avoid any challenge as they smell exactly the same as all the other bees in that box, and so will be able to enter and help themselves to food. They can then return to their own hive carrying that food, and the cycle frequently becomes repeated. The nucleus colony is seen by the forager-bee as simply an extension of it's own hive, and all that it (or they) are doing is re-locating food from one part of 'the hive' to another. It is a very 'civilised' form of robbing which appears to the beekeeper as normal entrance traffic, but the nucleus colony will almost certainly suffer as a result.
If, however, the beekeeper is unlucky enough to have included a forager-scout bee when making-up the nuc, then a much different outcome develops, as a Waggle Dance will be generated as before, and mob-handed robbing will undoubtedly result from this.
All the above cases have two solutions: the first is to move nucleus colonies to a separate yard, preferably more than three miles away. The second - if you operate within a single yard (as I do) - is to fit anti-robbing screens to every nuc as soon as it is made up. Even if a forager-scout (accidently included within a nucleus colony when first made-up) is able to circumvent that anti-robbing screen and thus 'report back', subsequent robber-scouts will not, and so any potential robbing soon fizzles out.
The 'trick' regarding robbing is to prevent it from starting - which is so much easier than trying to stop it once it's underway.
Hope at least some of the above is helpful.
LJ