[Neff]

In the UX world they are usually called urgency or scarcity triggers [1][2]. They also play in to the social default bias [3] people tend to have, where when alone you tend to do things that you know others have done.

1: https://www.researchgate.net/publication/259906084_Scarcity_... 2 (PDF): https://www.uvic.ca/socialsciences/economics/assets/docs/sem... 3: https://www.coglode.com/gem/social-default-bias