The answer to 'why is my roof vent sagging' is that a sagging roof vent is usually caused by structural issues or deterioration of the vent itself. This can happen over time due to factors like weather, age, or improper installation. Symptoms include the vent appearing crooked or uneven, and it may not seal properly around the roof opening.
Some common causes are rotting or damaged roof framing, loose or broken vent flashing, or the vent itself becoming warped or cracked. In many cases, the vent will need to be repaired or replaced to prevent leaks and further damage. If the issue is with the roof structure, that will need to be addressed as well.