IRB will not specifically say what triggers a tax investigation, however:
Sometimes it is obvious, like an omission picked up by IRB.
Sometimes it is random.
You may be in a IRB target area – IRB set up taskforces to target areas that are thought to be at high-risk of tax fraud. These maybe geographic and/or specific sectors, with property income being perhaps the most common.
Have you done something to attract the attention of IRB? This may be that your income is dramatically up and down or that returns are always late, or that you are claiming excessively high expenses in proportion with your income – all these things can make you high risk in the eyes of IRB.