Airpods Bluetooth interference when cycling
Bluetooth operates in the 2.4GHz band.  There are a number of other things that operate in this band:


WiFi
Wireless cameras
Wireless IoT devices
Bluetooth locating beacons
Cordless phones
Microwaves (though I’m fairly certain there’s not a lot of Hot Pocket preparation happening at traffic intersections)


Aside from the increased Bluetooth bandwidth congestion you’re likely to experience in crowded areas, you are also suseptible to bandwidth crowding from all the other potential sources of interference.  

From personal experience, when riding the subway in NYC, I found that my Bluetooth headset would occasionally become “static-y” especially around rush hour and clear when the train was empty.  A wired headset cured the issue.