Trump does this when he knows he's going to have to back down on something or concede some point to Iran. That way he can sell it to his base that rather than caving he brought Iran to the table with his belligerent threats. Good interview on the dynamic:
Josh Nathan-Kazis: Trump posted wild threats on Tuesday, saying “a whole civilization will die.” Then, late in the day, he announced a ceasefire. What do you make of the sequence of events?
Trita Parsi: You can go back to Tuesday morning and look at the tweet that I wrote, in which I said that there’s a likelihood that Trump is issuing these bombastic threats precisely because he’s going to cave, and so he needs to create a narrative that covers up that fact. By issuing these threats, he wants to leave the impression that it was the Iranians that caved because they were terrified. But when you scratch the surface and you see what was actually agreed upon, I think it’s very clear exactly what happened: Trump caved.
Poor taco.
https://www.youtube.com/watch?v=npjF032TDDQ
Because of this man the song will never leave my head.