possibly unusual spider behavior?
Those appear to be Daddy Longleg spiders, and they are known for forming spider clusters, which is considered as gregarious behavior. Quoting Wikipedia:


  Many species of harvestmen easily tolerate members of their own species, with aggregations of many individuals often found at protected sites near water. These aggregations may number 200 individuals in the Laniatores, and more than 70,000 in certain Eupnoi. Gregarious behavior is likely a strategy against climatic odds, but also against predators, combining the effect of scent secretions, and reducing the probability of any particular individual being eaten.