Question: Why are some instances defederating instances which don't sign the fedipact? ( kbin.social )

Something I don't understand currently about the whole Meta/Threads debacle is why I'm seeing talk about instances which choose to federate with Threads themselves being defederated. I have an account on mastodon.social, one of the instances which has not signed the fedipact, and I've had people from other instances warn me that their instances are going to defederate mastodon.social when Threads arrives.

I have no reason to doubt that, so, assuming that they are, why? I don't believe instances behave as any kind of relay system: anybody who wishes to defederate from Threads can do so and their instances will not pull in Threads content, even if they remain federated to another instance which does.

I'm unsure how boosts work in this scenario, perhaps those instances are concerned that they'll see Threads content when mastodon.social or other Threads-federated instances users boost it, or that their content will be boosted to Threads users? The two degrees of separation would presumably prevent that, so I can see that being a reason to double-defederate, assuming that is how boosts work (is it?).

Other than that, perhaps the goal is simply to split the fediverse into essentially two sides, the Threads side and the non-Threads side, in order to insulate the non-Threads side from any embrace, extend, extinguish behavior on Meta's part?

Ultimately, my long term goal is just to use kbin to interact with the blogging side of the fediverse, but there are obviously teething issues currently, like some Mastodon instances simply aren't compatible with kbin. I'm too lazy to move somewhere else only to move to kbin "again" after that, so in the short term I guess I'll just shrug in the general direction of Mastodon.

To be clear, I have a pretty solid understanding of why people want to defederate Threads (and I personally agree that it's a good idea), it's the double-defederation I'm not sure I follow. Is my understanding at all close? Are there other reasons? Thanks for any insight.