[tor-relays] Guard flag flapping

I have two relays on the same Gb/s connection. I followed the optimization tips offered in another thread, and think I have things running reasonably well. What I don't understand is why the Guard flag keeps flapping back and forth on both relays.

https://atlas.torproject.org/#details/89B9AE4C778DE44AFFAB791093E19979616E69...

https://atlas.torproject.org/#details/11A579CC6CEE644A390704977B70CBA8D83477...

If you look at the 1 week graph of weights, you'll see what I mean.

I haven't been able to find any documentation or explanation for this behavior. Anyone have any thoughts on why this might be happening, or any pointers on how to further investigate?