Yeah, I was surprised at how much it swings, but it's high enough it shouldn't matter (and easy enough to not use the cards for a month, let it rebound, then borrow whatever I need to borrow).

The report from my bank never says why. It does list factors that contribute to my score, but they're all "good" (low usage as % of available, all payments on time, etc). And never change.

I'm signed up for all the credit bureaus free accounts so I can freeze/unfreeze my credit. They send out reports monthly, along with one of my CCs. All of them have the reason. And yeah, the score is ~800 so it doesn't really matter. Still interesting to see how it moves with relatively small balance changes.