Bees are disturbed by lawn mowers operating near the hive during nectar (or pollen) dearth. In other words - when pollen is not getting produced by flowers. To avoid agitating the bees while mowing, create a grassless patch under and around the beehive stand using gravel and weed barrier fabric. You can also try pacifying the colony when you mow by smoking the hive and adding sugar water to the feeders.