For additional context, here are the two initial paragraphs from the New York Times article:
A peaceful march by Central American migrants waiting at the southwestern United States border veered out of control on Sunday afternoon, as hundreds of people tried to evade a Mexican police blockade and run toward a giant border crossing that leads into San Diego.
In response, the United States Customs and Border Protection agency shut down the border crossing in both directions and fired tear gas to push back migrants from the border fence. The border was reopened later Sunday evening.
And to repeat the phrase in question (emphasis mine):
The unrest in Tijuana comes amid broader discussions.
First, tenses can be mixed without a problem. This can be done between paragraphs (as in this case) and even within the same sentence, if done appropriately. Not mixing tenses is merely a rule of thumb—because it often results in something ungrammatical or strange—but it's not a necessity. There's nothing inherently ungrammatical about it.
I still have some of the cake that I bought last week.
In this case, although an incident took place in the past, and there was unrest in the past—it's also true that there still is unrest.
If the past tense had been used, the reporting would be both less meaningful and also somewhat misleading. It might be interpreted as implying that there is no longer any unrest and that things are now fine.
By using the present tense in this section, the ongoing unrest is being highlighted.