While TikTok is not very forthcoming with details about how its algorithm works, there are many theories on what works (some more logical than others). One (more logical) theory is that whenever somebody adds a video to TikTok, it is pushed to a group of viewers. The algorithm will then look at the percentage of the video the initial viewers watched and the number of likes, shares and comments it received. If these are high enough, TikTok will continue to push the video to more viewers.