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Probably pheromones in the soil or the natural urge to investigate other species nests. Just think that the bees see freshly dug soil and may think an interloper is making a nest site
Wild Flower Society member
A couple of possible suggestions:
1) you are digging iover their existing nest site and they are struggling to orientate to find their nest burrows
2) Your visitors may be Red Mason-bees (Osmia rufa) which use damp soil (preferably with a clay content) for making the seals on their brood cells and for the nest closure plug.
Lat/Lng: 51.8, -1.0
OS grid ref: SP7516