Nearby three or four smaller bumblebees are checking out flowers at a rate of three or four to every one the larger bee visits. While this larger bee visits the flowers via the 'front door' the smaller ones buzz at reckless speed to the 'back entrance'. Each flower has a small hole that has already been opened up to give direct access to the nectaries. I recently watched bees using the same technique on aquilegia. There's a dribble of sugary nectar from each hole.
Some of the leaves below the flowers are turning a darker, sooty, shade with lighter spots which I guess is caused by mildews and moulds growing on the spilt nectar. A similar thing happens when aphids feed on the underside of sycamore leaves, drizzling honeydew onto the leaves below.