Checked the attic, there is a lot of space above the room. The entire attic is covered with insulation, so I didn't walk all way above the room. I did notice there is some crack along the bulging line. I added an image
There is thing called truss lift, where in the winter the trusses actually lift in the center of the house. It doesn't happen in every house even when the next house is the same as the one in question.
If you google truss lift you can find all kinds of explanations non of which make much sense but.
Most times the drywall will put the ceiling up first and add no screws near the interior wall so that if it happens the drywall can bend a little and the corner is not effected.
But even then there isn't much of a chance to hide it in a narrow hallway if the trusses lift.