I think I solved it. I remembered that I have Veeam running on a Windows PC that backs up every night at 12:30. Looks like last night was a rather large job. The target for that backup is a share that utilizes the cache for speed, and then the mover moves it off the cache. But since it's an overnight backup, I will move the target to a new share that doesn't touch the cache at all.
Thanks for helping me discover my other issues though. I definitely need to sort out my zfs issues.