How exactly does gaining mental trauma work?

It says you gain a mental trauma when you increase to the next degree of madness (page 234). Does this mean as you increase to the next line on table 8-5 eg from 0-9 to 10-19 insanity points, then 10-19 to 20-29 insanity points? Or does it mean as you move from Stable to Unsettled, then Unsettled to Disturbed? Its isn't very clear.

Also, what is the Disorders column of that table for?

You may get a trauma for advancing to the next row of the table (essentially, every 10 insanity points). You will get a disorder for every category you rise, with the first disorder at 30 points.

Apologies, I meant mental disorders.

Are these at the discretion of the GM at every 10 points of insanity and comulsary at 1st Minor stage on the table then?

No. As I said, you get a mental disorder at 40 (mixed up where the categories start), 60 and 80 insanity points. Every time you reach a full 10, you roll a willpower test with the Trauma Modifier given in table 8-5. If you don't manage to make that test, you get a mental trauma, one of those from table 8-6. It's pretty straightforward and well-explained in the rules of that page, actually.