Homeowner's insurance underwriting inspections are ordered by insurance companies under several circumstances:

New Insurance Application
When new homeowners seek to acquire insurance the insurance company will send an inspector to the property. The insurance company needs "eyes in the field" to verify and note specifics about the property.

Book of Business
Sometimes insurance companies will place their "book of business" on a timed inspection schedule as in every three years. In this case they are looking for possible changes to the building and or grounds.

Event Driven
Finally, an event might trigger and inspection. These events could be an addition or modification to the property, the insured has requested more insurance or a claim has been filed.

Major and minor "conditions":

Majors
Some of the items inspected will trigger cancelation of the policy if they are found to be inadequate, delapidated or dangerous. These are referred to as "major" conditions.

Minors
Other less important items will trigger recommendations for repair or correction from the insurance company. These are referred to as "minor" conditions.

What triggers a "major" or "minor" condition during a homeowners insurance inspection?

There are many facets of the building and grounds that are inspected and noted for the insurance company records at the time of an inspection. See the list of conditions.