There's probably at least three different kinds of when we do this: (1) when a subthread is off topic but the parent comment isn't; (2) when a subthread is a step into flamewar or into more generic discussion, or otherwise breaks the site guidelines; (3) when a comment was posted as a child but isn't replying to anything specific in the parent—this happens a lot, especially when the original parent is the top comment at the time.

Edit: oh also (4) when a subthread is particularly good and on-topic and there's some problem with the parent (e.g. it's downvoted or downweighted) that is preventing the good subthread from getting the attention it deserves.

Edit: oh, also (5) when a subthread is extremely top-heavy and we're trying to prune it to make the tree more balanced.

Often I'll post "We detached this subthread" (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...), but not always. Especially if it's a #3.