Without Googling Rio Vista I'd contribute the following.
Almond groves, very prominent in Cali, "rent" honey bees. On occasion, when a hive grows to what the bees deem "too many" the bees will SWARM. That means that up to half the hive will leave to start one of their own. During this period of movement, they are surprisingly docile. They do not have a "home" to defend. More that likely they do not have a queen. They'll "make one", almost literally, when they find a home.
Bee keepers love this event. It represents a free hive! Free bees! You gotta live it!
Glad you didn't have anything worse than that happen. Like stings or laying down the bike due to lack of "vision".
Ride on, Bro. It doesn't happen very often. But, in Cali "farm" country there's lots of bees facilitating pollenization.