Why is my 830 downloading activities from Garmin Connect?

About two weeks ago, my 830 started DOWNLOADING activities from my Garmin Connect website account. The activities downloaded to the 830 are activities recorded with my 800 device. When I view history on the 830, it shows activities recorded on the 830 AND activities recorded on the 800 and these 800 activities have a bicycle icon beside them.

I use my 830 and 800 on two different bikes. Until two weeks ago, each device maintained its own activity history. But now the 800 activities are downloading to the 830 when the 830 connects to the Garmin Connect website to upload the 830 activities. The 800 does NOT download the activities recorded on the 800.

Why did this activity downloading to the 830 start, and how do I make it stop?