Appeal to Popularity combined with Appeal to Consequences; here, it's suggested that because something is becoming popular, it should be accepted quickly or the person being spoken to will lose out in the long run. The name comes from the classic idea of getting on the bandwagon before it leaves; in this fallacy, the fact that there are a lot of people on the bandwagon and it might leave are the only reasons given to accept, with no reason why getting on the bandwagon is actually a good idea.
This is referred to as Fear of Loss in sales; a salesman will claim that he's only allowed to sign up a certain number of people to a fantastic deal and has already got most of his quota for today, so if the person he's speaking to doesn't act they stand to lose out.