Hype trains on Twitch can now be triggered by high chat activity (based on the streamer's individual settings)
https://x.com/TwitchSupport/status/1934719301816062168

If you don't want this new change you can apparently revert it in your hype train settings here
https://dashboard.twitch.tv/monetization/community/hype-train

And this Twitch help article has a rundown on all of the things you need to know about hype trains
https://help.twitch.tv/s/article/hype-train-guide