We don't have a list of specific reasons at the moment, but generally, instances on our blocklist are there due to some kind of abuse. The same is true for the one you mentioned - have a look at these stats:
It's an instance with 0 active weekly users, 10 posts and 3 comments, but they have 16200 accounts on their instance. In other words, there's an extremely high likelihood that the instance is absolutely full of bot accounts, and by defederating, we ensure that these bots can not be activated against lemm.ee in any kind of spam attack or vote manipulation.