Note: I have no evidence for this, it's just me ranting based off info I know. Feel free to correct me in the comments. Peccaries are relatives of pigs that evolved in the early Miocene of what is now Mexico(?). Now, people that know a lot about modern extinctions probably know that if you have an extinct flightless bird species, it was probably killed by pigs or rats eating their EGGS (examples: dodos, Moa probably, etc.). The usual cause of the extinction of the (large) terror birds was the introduction of north American predators, such as Smilodon. However, this is not the case, as 1: Titanis existed, which was one of the largest Terror Birds that moved into in North America, which had, you know, Smilodon, and 2: the other large south American terror birds went extinct before the land bridge connecting the Americas was even fully formed!!! Now, back to Peccaries. Since Terror Birds are, you know, birds, they need to lay eggs. However, Terror Bird were flightless, so they had to resort to laying them on the ground, where they were easy pickings for the Peccaries. This, combined with competition with the land crocs and Sparassodonts, along with environmental changes, lead to the extinction of the larger terror birds.
Wikipedia and my brain i guess
