Tangentiality is the tendency during speech to talk about topics unrelated to the main topic of discussion. While many people engage in tangentiality occasionally, tangentiality may indicate an underlying mental health issue, particularly psychotic disorders

 
A normal person goes off on tangents sometimes. And this can help the topics sometimes and keeps the talk going on. However, constant tangentiality may indicate a mental health condition.

People suffering from mental disorders may be so focused on their own internal dialogue that they cannot focus on conversation or questions; and they may give irrelevant answers to questions.

Tangentiality is not only present in psychotic disorders, it is also common during times of anxiety. People experiencing anxiety may become tangential by talking about seemingly unrelated topics

Post-traumatic victims may go on tangents either to avoid discussing the trauma or because they are unable to think clearly.

To sum up, causes of tangentiality:

• Normal with some people, specially if occasional
• Psychotic disorders
• Anxiety disorders
• Post-traumatic state