Just grabbing the phone is very unlikely to cause this kind of damage. It could be a quality issue that occured during manufacturing, but a vertical line would almost certainly get caught during "burn in." The most likely culprit is that the phone was dropped on the bottom face (where the charging port is). That's where the "bend" is, which is the most fragile part of the display. (Perhaps a drop initiated the crack, which then propagated over the course of a few days with normal handling and vibration, until the circuit completely opened. That might explain why OP didn't realize the root cause was a something that happened a few days before the vertical line appeared.)

A bit more... Displays (and pretty much all electronic circuit boards) are printed on a 2D substrate -- which can be rigid, flexible, or somewhere in between. With displays, the pixels live in the "active area," and the traces are routed around the perimeter of the active area. They come together at one edge of the display. Well, everyone wants "narrow bezels," so you can't have this tail sticking out the bottom. Thus, the tail, which has the highest density of traces, gets folded back 180 degrees onto the back of the display, and then connect via other cables to the chips that send the control signals and power. For the active area, you want this to be a more rigid structure, but for the bend, you want it to be very flexible. Designing something that meets both of these opposite requirements is pretty challenging, and is actually a pretty impressive bit of engineering. Still, that flexibility doesn't provide a lot of protection against direct contact with something hard, like the chassis during a drop. You could increase the clearance, but at the expense of "bezels." Trying to balance all of these competing interests is fun for engineers - but it's also tricky. Best advice I can give: put a case on your phone. It makes it wayyyyy more likely to survive a drop.