The reason you that Social Security is sending you for mental testing depends on the type of exam.
If there is an indication of memory problems (which may require the use of a WMS, or Wechsler memory scale), borderline low intellectual functioning, or a premorbid decline in IQ (which may be due to organic brain syndrome or head trauma), a disability claimant may be sent to a full psychological evaluation, such as a WAIS, or wechsler adult intelligence scale.
Children requiring mental testing or mental evaluation are often sent to the following types of exams when the needed:
If you are applying for Social Security disability or SSI disability for a physical impairment, and there is any indication on your application for benefits that you have depression or anxiety, Social Security may request that you go to a MSE, or mental status exam. This may be the case even if you do not allege this condition as the basis for your disability on your application. In fact, it is very frequently the case that disability claimants will be sent to an MSE simply because their family doctor made one single mention of anxiety or depression somewhere in their treatment notes. Disability examiners for Social Security (much more so than a disability judge) are required to investigate the "potential" for a claim based on a mental impairment if any indication--even the slightest indication--of such a mental impairment exists.
In addition, if you apply for disability for a physical problem such as chronic pain but the doctor can't find anything wrong with you, you may be required to attend a mental status exam.
Disability claimants who apply on the basis of depression or bipolar disorder, or a psychotic illness like schizophrenia, may be sent to full psychiatric evaluations conducted by a psychiatrist to assess the severity of the condition.
Learn more about Social Security Disability for Mental Impairments