1. When you try to register with multiple numbers in a short period.
2. Sending Lots of Messages in a short period of time.
3. A sudden increase in the number of messages sent Especially on a new number.
4. Sending the same message to lots of people, in a short period of time.
5. The ratio of sent v/s received is wrong. You are sending lots of messages but you receive very less number of messages.
6. People report or Block you. This is the #1 reason for your number getting banned.
If you believe that you have not violate the terms of the WhatsApp, you may contact them: firstname.lastname@example.org