Incidental and unintended data leakage.
TikTok is a virally popular video app used globally by a generation of adolescence that have normalized the behavior of recording inane happenings in their life multiple times a day, with an undeveloped sense of propriety and limited ability to fully comprehend the significance of their actions.
Not that I have much of an opinion on TikTok's usage as an intelligence gathering source, I certainly appreciate its potential as one.
My daughters are constantly taking videos all over the place, with little regard to what is in the video beyond themselves. Even though they've been hounded on what is and isn't appropriate to capture in a video, I'm sure they have an under-developed sense of security/concern in that regard, particularly for "private" or deleted videos.
Access to the full corpus of video data uploaded to Tiktok would be a gold mine, least of which would actually be my adolescent daughters. Unrelated conversations picked up in the background, where/when they took the video, compromising/confidential artifacts exposed incidentally in the background, facial recognition on individuals in the video (whether the subject of the video or otherwise), etc. There are an immeasurable number of ways such a rich data source could be mined for signal intelligence.
Note that TikTok is far from the only app that fits this definition. There are plenty of other platforms that create a similarly useful corpus of video data. But anecdotally from my kids and their friends, TikTok is the flavor of the moment. Especially so for those young enough for the above concerns (or lack thereof) to be most valid.