ES cluster always auto-balancing

We turned auto balancing on in the cluster, and we started seeing that there were always 9 shards moving around (9 is the maximum number of shards that we configured).

Our nodes were already quite balanced (we eye-balled them and they had similar shard counts and free space). I took a closer look and I saw shards moving from nodes with more free disk space to nodes with less free disk space.

Why is ES taking a decision like that? Is there a way to look into why ES decides to balance a shard in a certain way?

We are currently using ES 6.8


I will cross-post in StackOverflow:

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