There are four so far, and how exactly they trigger remains unclear. Nevertheless, it seems like either one or both of the big bads of the two upcoming expansions are being teased, and fans think they know who the dastardly demons are. Spoilers for Diablo 4's Campaign to follow!
As compiled by website Wowhead (where you can view each), the first three trailers are jump-scare-style short clips which show a threatening demon that is believed to be Mephisto, the Lord of Hatred. This would make sense, considering the role that particular Prime Evil plays during the course of the campaign, and could tie up some of the lingering plot threads that remain.
The fourth, however, is something else entirely, a horrific demon-dragon thing; the theory here is that this is Lucion, Mephisto's son, and brother to Lillith, the primary antagonist of Diablo 4. It's quite compelling, actually, and we don't recall seeing any such stingers after our own initial playthrough