OK, that's gonna be rather speculative, but the series kinda is rather enigmatic. Also, this is all like one big spoiler.
Lets start with "why did it open?". It was probably because of power output needed to send Kamala into the past.
Looks like there was an interplay between Aisha, Kamala and Najima. Aisha was trying to get help from a future user of the bangle. That caused Kamala's visions of the past, but didn't quite work out. Still, as Najima touched it, the vision not only came back, but even Kamala's grandma saw it. Seems this was a dance for three (or maybe even four...) - just after Najima stabbed the bangle, Kamala was finally sent back to past. This made a hole in our reality and apparently also in the veil.
Now, what about closing it?
It's hard to say what Najima could do and how. Like other jinn she had some connection to the Noor. Now this breach was a very big connection that she apparently could manipulate. So, what happened when she touched it? Her body was destroyed, but she managed to manipulate the breach and even influence Kamran at a distance. But what about "dying"? While perhaps she died, in a way, I think it was rather her succeeding in going back to Noor, leaving remains behind, while spirit, or actual "jinn" part, left where she wanted to go.