I'm sorry, but I'm afraid I don't remember. Mostly we do that when a subthread goes too offtopic, especially when it makes the discussion more generic, which is a bad thing on HN [1]. But we might also do it as a pruning mechanism because the thread has become top-heavy and a subthread can stand on its own. We also do it when a comment is not really a reply to its parent, but was posted there for some other reason (usually because the parent was the top comment). I take your word for it that the latter was not the case here.

Most likely it was because the subthread was too generic relative to the OP. What makes articles intellectually interesting are the specifics, the curious bits, the diffs [2] between it and comparable things. In the present case that was certainly the Ugandan aspect, and the theme of American cultural domination vis-a-vis Europe is a change of subject relative to that.

Changes of subject can be fine if they're unpredictable and whimsical, but definitely not when they're a move towards the generic. Generic topics are like large planets with powerful gravitational fields—or even black holes—the default trend is for all threads to get sucked towards them, and that leads to a much less interesting place.

[1] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...

[2] https://hn.algolia.com/?dateRange=all&page=0&prefix=false&so...